In a dry pan over low-medium heat (no oil), lightly toast the pine nuts or any other nuts you’re using. Remove from heat, allow to cool.: Right away you will notice the nuts releasing a warm, toasty aroma that signals they are ready; the sound is subtle, and the color shifts to a pale golden. Toasting deepens the flavor and adds a crunchy background that blends into creaminess when processed, which is why I never skip it. Watch them constantly because once they begin to color they can quickly burn, tasting bitter instead of sweet. If you over-toast a few nuts, pick those out and continue with the rest to avoid bitterness. Let them cool completely on a plate, because warm nuts can slightly wilt the basil if added too soon, and that changes the texture of the final pesto.
Place all of the ingredients except for the olive oil in the bowl of a food processor. Blend until completely broken down, scrape off the sides of the bowl as needed.: As the blades spin you will hear a steady chopping rhythm, and visually the mixture will transform from leaf pieces to a coarse, herbaceous paste; the garlic will integrate and the cheese will marry with the nuts. The purpose here is to achieve an even, broken down base that lets the oil emulsify well later. If you rush this step the oil will separate or the pesto will be uneven in texture. A frequent error is adding oil too early; that can make the mixture slick and prevent the blades from effectively breaking down the solids. Use a spatula to scrape down the sides periodically so everything gets incorporated, and stop once there are no large chunks left, but before the paste becomes overly liquefied.
Add the olive oil gradually with the food processor running until you get a slightly emulsified sauce. Thin out with extra oil or water if needed.: When you stream in the extra virgin olive oil you should see the paste shine and glide, and the sound of the processor will soften as it becomes more fluid. This slow addition encourages a gentle emulsion that yields a creamy mouthfeel rather than an oily slick. The why is simple, the oil needs to be incorporated progressively so it suspends in the herb-cheese matrix. Troubleshoot by checking the texture; if it appears too thick, add more oil or a tablespoon of cold water to loosen it, but add little by little to avoid over thinning. If the pesto tastes oily, it likely needed more solids or less oil, so next time reduce the oil slightly.
