Heat oil in large nonstick skillet over medium-high heat. Add onions; sauté until golden, about 5 minutes. Add squash; sauté 4 minutes. Add broth, jalapeño and garlic; bring to boil. Cover; cook until squash is almost tender, about 5 minutes. Stir in coconut milk, lime juice and curry paste. Simmer uncovered until squash is tender and liquid is slightly reduced, about 4 minutes. Season to taste with salt.: When the oil begins to shimmer slightly, you will smell a faint fruitiness, and the pan will look glossy. A properly warmed pan ensures that the onions sizzle gently rather than stick, producing a sweet, caramelizing aroma that forms the base of the dish. If the oil smokes, lower the heat immediately; overheated oil can burn aromatics and leave a bitter taste. A common mistake here is to rush and start with too high heat, which causes uneven browning and scorch marks. The sizzling sound will be steady and moderate, not a fierce crackle, and the pan surface should show thin waves of movement in the oil as it warms.
Meanwhile, cook noodles in large pot of boiling salted water until just tender but still firm to bite. Drain noodles. Return to pot. Add squash mixture and cilantro to noodles; toss to blend. Serve.: As the onions soften, they release moisture and then sugars that begin to turn golden, giving off a warm, slightly sweet scent. Stir occasionally so the edges brown evenly, scraping any browned bits from the pan to capture flavor. The sound changes from a sharp sizzle to a softer pop as the moisture reduces. If the onions are browning too quickly, reduce heat, because burnt edges will bring a bitter undertone. Aim for translucent centers with golden edges, which will meld seamlessly into the sauce later.
Add squash; sauté 4 minutes: Adding the cubed butternut squash introduces a denser texture that will start to sear slightly at the edges, giving faintly nutty notes. You should hear a gentle, steady sizzle as the squash contacts the oil, and the pieces will begin to pick up a light golden blush. This initial saut e9 helps develop flavor and caramelization before adding liquid. Avoid overcrowding the pan, because crowded pieces steam instead of browning, losing that toasty depth.
Add broth jalapeño and garlic; bring to boil: The moment the vegetable broth hits the pan, it lifts the browned bits and creates an aromatic steam. The minced jalapeño adds a fresh, green heat while the garlic becomes fragrant, giving off a warm, savory perfume. Bringing the mixture to a gentle boil releases those aromas and readies the squash to absorb flavor. A common pitfall is adding garlic too early on high heat, which can burn and taste acrid; adding it now lets it mellow as it simmers.
Cover cook until squash is almost tender about 5 minutes: Covering creates steam that penetrates the squash, softening it quickly and evenly. Lift the lid to check doneness, piercing a cube with a fork; it should yield but still retain some structure. You will notice the steam carries the curry scent, and the liquid reduces slightly under the lid. Avoid overcooking during this stage, because fully collapsing cubes will break down too much when you finish simmering uncovered, resulting in a mushy sauce.
Stir in coconut milk lime juice and curry paste: Adding the coconut milk envelops the pan in a rich, creamy aroma while the Thai red curry paste blooms in the fat, deepening color and complexity. The lime juice adds an immediate bright note that cuts through the richness. Stir thoroughly so the paste disperses, transforming the liquid into a glossy, saucy coating for the squash. If the curry paste clumps, press it against the pan to dissolve; clumps mean uneven flavor pockets in the final dish.
Simmer uncovered until squash is tender and liquid is slightly reduced about 4 minutes: With the skillet uncovered, the sauce concentrates, becoming slightly thicker and more flavorful. You will hear a gentle simmer and see the sauce cling to the squash. The aroma will deepen, and the color intensifies to a warm orange. Taste as it reduces so you can stop when it reaches your preferred consistency. A typical mistake is letting it reduce too much and dry out, so maintain a low simmer and add a splash of broth if needed to rescue the texture.
Season to taste with salt: Salt brightens and rounds the flavors, making the coconut and curry paste sing. Add a little at a time and taste after stirring so the seasoning layers evenly. Proper salting transforms the dish from flat to complex. Be cautious not to oversalt; because the broth and paste contain sodium, gradual additions are best to avoid an overly salty final bowl.
Meanwhile cook noodles in large pot of boiling salted water until just tender but still firm to bite: The pot of boiling salted water should smell faintly briny, and the noodles will turn glossy as they cook. Cooking until al dente means they will have a slight bite that helps them hold up when tossed with the saucy squash. Overcooked noodles become mushy and can soak up too much liquid, leaving the dish heavy. Keep an eye on the clock and test early to ensure the perfect texture.
Drain noodles: Draining should be swift to avoid residual cooking from trapped steam. A good drain leaves the noodles moist but not waterlogged, ready to accept the curry sauce. If you rinse, which I do not recommend here, you risk washing away the starches that help the sauce cling. If the noodles seem sticky after draining, toss briefly with a teaspoon of oil to separate them.
Return to pot Add squash mixture and cilantro to noodles toss to blend: When you add the warm squash mixture to the noodles, steam will rise in a fragrant cloud, and the noodles will soak up the saucy coating. Toss gently so each strand or grain gets enveloped, and fold in the chopped cilantro so its herbaceous scent stays bright. The combined textures should be silky sauce meeting tender squash and springy noodles. Avoid vigorous stirring that can break the squash into mush; gentle folding preserves the integrity of the cubes.
Serve: The final presentation should be glossy, warm, and aromatic, with flecks of green from the cilantro and a visible sheen on the sauce. Serve immediately so the noodles and sauce remain at their best texture. A common mistake is to let the dish sit too long, which allows noodles to absorb too much sauce and the squash to cool, losing vibrancy. Serve hot for optimal taste and mouthfeel.