Preheat oven to 400 degrees.: As the oven warms it creates a stable environment for the tomatoes and feta to roast evenly, developing deep caramelized flavors and gentle char. You should feel the kitchen temperature begin to shift and smell faint warmth from the oven; that steady heat is important so the tomatoes soften rather than steam. A common mistake is sliding the dish into an oven that is not fully hot, which leads to uneven roasting and less jammy tomatoes. Make sure the rack is positioned mid oven before you begin.
Add grape tomatoes and garlic to a 3 quart baking dish. Toss with olive oil and a few pinches of salt and pepper. Place feta in the center of tomatoes, turn to coat with oil.: When you toss the grape tomatoes with olive oil , the oil helps encourage browning and carries the aroma of the garlic . Placing the feta block in the center creates a communal melting point so the cheese softens into the tomato juices. You should notice the oil glistening on the tomatoes and the garlic smelling fragrant and warm; this pre roasting mix sets flavor expectations. Avoid piling ingredients too high, which prevents hot air from circulating and inhibits proper roasting.
Bake in center of preheated oven for 30 minutes.: During this time the tomatoes will begin to burst and release concentrated juices, and the feta will start to soften and warm through. Listen for faint pops from the tomatoes and watch for skin split and juices pooling, which are signs they are roasting correctly. This initial bake builds the sauce base and sweetens the tomatoes. A mistake to avoid is opening the oven repeatedly, which drops the temperature and extends cooking time unnecessarily.
Then remove the baking dish, raise oven rack up to upper 1/3 of the oven. Return baking dish to oven and increase oven to 450 degrees. Bake until tomatoes and cheese are starting to brown slightly, about 10 minutes longer.: This step intensifies caramelization and encourages a little browning on the feta , adding a toasty complexity. Visually you want golden spots on the cheese and deeper color on tomato skins; aromatically the dish will smell richer and nuttier. Beware of leaving it too long at the higher temperature or you will get bitter char; check frequently near the end.
Meanwhile boil pasta to al dente in salted water (I use 1 Tbsp salt and about 3 or 4 quarts of water), according to time listed on pasta package.: While the oven finishes, the pot should be at a rolling boil so the pasta cooks evenly. Seasoning the water with a tablespoon of salt seasons the pasta from the inside out, which is critical since the sauce is simple. Cook to al dente so the pasta keeps shape when tossed with hot sauce. Overcooking leads to a mushy final texture and undercooking gives a starchy, raw bite.
Reserve 1/2 cup pasta water and right before draining pour and submerge spinach in with the pasta (it should wilt almost instantly). Drain pasta and spinach.: The reserved hot pasta water contains starch that will marry the roasted mixture into a silky sauce, so keep it handy. Submerging the spinach briefly wilts it quickly, preserving color and preventing excess water from diluting the sauce. After draining, the pasta should be warm and the greens soft but bright. A common error is discarding the pasta water; without it the sauce may be too thick or clumpy.
Smash the tomatoes, garlic, oil and feta together with the back of a wooden spoon. Toss pasta in with mixture along with basil, parsley and red pepper flakes.: At this tactile moment you will feel the tomatoes collapse and the feta give way, releasing a fragrant, glossy sauce. The perfume will be a mix of roasted garlic , concentrated tomato sweetness, and salty tang. Using a wooden spoon allows a bit of resistance as you mash, which helps integrate solids into a cohesive sauce. If you skip thorough smashing you may end up with large unincorporated chunks and uneven coating on the pasta .
Thin mixture with a few tablespoons of pasta water at a time to reach desired consistency. Season with salt and pepper as needed. Serve warm.: Add the hot pasta directly to the baking dish so the residual heat helps the sauce cling to each piece, and fold in aromatic herbs like basil and parsley for lift. You should hear a gentle tinkle as pasta meets the saucy pan and see the strands or shapes become glossy with coating. The red pepper flakes should release a faint warming scent. If you mix too vigorously you can bruise delicate herbs, so fold gently to distribute evenly.
Thin mixture with a few tablespoons of pasta water at a time to reach desired consistency. Season with salt and pepper as needed. Serve warm: Adding warm reserved pasta water bit by bit will loosen and emulsify the sauce into a silky finish that clings to the pasta . Taste as you go and adjust with salt or pepper; the feta is salty so little may be needed. The final dish should be glossy, slightly saucy, and warmly aromatic. Over thinning will make the dish soupy, while under thinning leaves it clumpy, so aim for a smooth coating that moves when the bowl is tilted.