Cook pasta to al dente in a large pot of salted boiling water according to package directions. Before draining pasta reserve 1/3 cup pasta water.: The aroma of boiling water and the hiss as the dry pasta hits the pot is the first sensory clue that the dish is underway. You'll want the pasta to be tender but still have a firm bite, because it will continue to soften as it cools and when dressed with sauce. Listen for a gentle rolling simmer and watch the water return to that boil after adding the pasta . Stir occasionally to prevent sticking, especially early on. A common mistake is overcooking, which leads to a mushy texture and a salad that absorbs too much dressing. If you are unsure, start checking at the lower end of the package time and test a piece; it should have a slight chew and a tiny white center when bitten.
Drain pasta then spread out onto a lightly greased 18 by 13-inch baking sheet to cool for 15 to 20 minutes.: That warm starchy water smells subtly of cooked grain and it performs like magic in blending sauces. The reserved water helps thin and loosen the pesto so it evenly coats the pasta without adding more oil. Scoop it out with a measuring cup just before you drain, and keep it nearby. The common error here is forgetting it, which can leave your pesto too thick or clumpy; if you forgot, warm water can be used sparingly, but it will not have the same gloss and binding power.
Meanwhile make the pesto: to a food processor add basil leaves, parmesan, pine nuts or walnuts, and garlic. Season lightly with salt then pulse until ingredients are minced. With food processor running slowly pour olive oil in through the feed tube and blend until nearly smooth.: Spreading the pasta on a sheet helps steam escape so the pieces cool without becoming mushy, and it prevents clumps. You'll notice the steam rising off the pan and the pasta losing that hot, sticky sheen as it cools. Move it once or twice so it cools evenly. A rookie misstep is piling it into a bowl while hot, which traps steam and softens the shapes too much, making the salad limp.
In a large bowl toss together drained pasta, arugula, tomatoes, mozzarella pearls, lemon juice and zest, and pesto. Season with salt and pepper to taste and add parmesan cheese to taste.: to a food processor add basil leaves, parmesan, pine nuts or walnuts, and garlic : When you pulse these ingredients, the scent of crushed basil and garlic will fill the kitchen; it is one of the most vivid sensory moments. Pulse until the ingredients are minced and display a coarse, emerald texture. The mechanical action releases essential oils from the basil and nuttiness from the nuts, which is why you do it first. Be mindful not to overblend to a completely uniform paste if you prefer texture; overprocessing can make the pesto flat and warm quickly due to blade friction.
Thin with reserved pasta water as needed. Store in fridge in an airtight container up to 1 day.: Salting early helps the blades break down the plant structure and brightens the flavors. You'll hear the motor change pitch slightly as the mixture becomes more cohesive. Season conservatively at this stage because the parmesan already contributes salt. A typical slip is oversalting before tasting; you can always add more later once the salad is combined.
With food processor running slowly pour olive oil in through the feed tube and blend until nearly smooth: Slowly adding the olive oil creates a silky emulsion that coats both the herb mixture and the pasta evenly. You'll see the color deepen to a glossy green and the texture become smoother but still slightly textured. If you dump the oil in too quickly, the mixture may not emulsify properly and can separate. If separation occurs, add a tablespoon of reserved pasta water and pulse to bring it back together.
In a large bowl toss together drained pasta, arugula, tomatoes, mozzarella pearls, lemon juice and zest, and pesto: At this stage the aromas are layered, with citrus brightening the herbaceous quality and the mozzarella cooling the mix. Toss gently so the arugula keeps some structure and the tomatoes stay intact. The warm pasta will help the pesto melt into the shapes, coating them evenly. A frequent mistake is vigorous tossing that bruises the greens and breaks the tomatoes, creating a wetter salad.
Season with salt and pepper to taste and add parmesan cheese to taste: Now is when you refine the seasoning, tasting for balance between acid, salt, and richness. Freshly cracked black pepper adds a final lift. Add the grated parmesan gradually, because a little goes a long way in deepening the savory profile. Over-seasoning is easy here, so taste between additions to avoid overpowering the fresh components.
Thin with reserved pasta water as needed: The reserved water will marry the pesto to the pasta , creating a glossy, cohesive coating. Add it sparingly while tossing, watching how the texture changes; a tablespoon can transform a clumpy mix into a silky salad. If you add too much, the salad will become watery and lose intensity, so proceed with care.
Store in fridge in an airtight container up to 1 day: Chilling calms the flavors and lets them marry, but the salad will change texture as the pasta absorbs dressing. Keep it airtight to preserve brightness, and bring to cool room temperature before serving if you want the oils to loosen. A common error is leaving it out too long, which can soften the greens and reduce the freshness of the tomatoes.