Cook spaghetti in a large pot of boiling salted water, stirring occasionally, until al dente; about 9 minutes. Drain, reserving 1 cup of pasta water.: As the water comes to a rolling boil, the scent of the salted steam will fill the kitchen, an encouraging sign you are on the right track. While the pasta cooks, listen for the gentle rolling of bubbles and the occasional softer sound as the strands shift, signaling that they are tenderizing. When you bite a strand, it should offer slight resistance at the center, that pleasant chewiness that catches the sauce. The reserved pasta water is golden and slightly cloudy, full of starch, and it is the secret glue that will help your pesto become a glossy, clingy sauce. A frequent error is discarding that water; without it your pesto can sit on top of the pasta rather than marry with it.
In the body of a blender or food processor pulse almonds, parsley, scallions, oil, lemon, garlic, and asiago; blend until smooth. Season with salt and pepper.: When you add the slivered almonds to the processor, they release a toasty aroma that deepens as you pulse. The first few quick pulses will break them down, and then adding the parsley and scallions will unleash a fresh, green fragrance that brightens the room. As you stream in the extra-virgin olive oil , you will notice the mixture transform from a coarse mash to a creamy, glistening pesto. The grated asiago melts into that fat, giving the sauce a silky mouthfeel and savory depth. Keep an eye on texture; overblending can make the mixture too smooth and warm, which dulls the herbal snap. If the pesto seems dry, a tablespoon of reserved pasta water can open it up. A common pitfall is adding all the oil at once, which prevents a proper emulsion; pour slowly for best results.
Toss spaghetti and pesto in a large serving bowl. Add reserved pasta water by the tablespoon until saucy. Season with salt and pepper. Serve at once.: Right after blending, the aroma will be layered and complex, but seasoning is where the flavors cohere. Add salt sparingly at first, because the asiago carries saltiness. Freshly ground black pepper adds a floral warmth that lifts the parsley and lemon. I always taste before final seasoning, because the salt level can change depending on how much cheese and reserved pasta water you added. Over-salting is hard to fix, so proceed cautiously and adjust incrementally.
Toss spaghetti and pesto in a large serving bowl: When you add the hot spaghetti to the pesto, you will feel a satisfying warmth as the sauce clings to the strands, releasing savory steam. Tossing by hand gives you control over distribution so every bite gets an even coating. The visual cue is a glossy sheen over the pasta, with flecks of green and tiny almond pieces. If the pesto sits on the surface without adhering, it usually means you need a splash of reserved pasta water to help the emulsion. Avoid tossing too roughly, as that can break the pasta and create a mushy texture.
Add reserved pasta water by the tablespoon until saucy: The reserved pasta water is the difference between a loose dressing and a sauce that hugs the spaghetti . Add it tablespoon by tablespoon, watching the sauce go from thick and pasty to smooth and silk like a ribbon. Each addition changes the mouthfeel, so pause to taste and look; the right consistency is glossy, not watery. A common misstep is adding too much water at once, which dilutes flavor and requires more cheese or seasoning to rebalance.
Season with salt and pepper. Serve at once: The final seasoning round should feel like the punctuation at the end of a paragraph, tying everything together. Once plated, the steam will carry the citrus, herb, and cheese aromas to the table. Serving immediately preserves the bright green color of the parsley and the texture of the spaghetti . If you wait too long, the pesto can oxidize and darken, and the pasta can slump as it cools, so aim to serve right after tossing.
