To make the pesto, zest and juice the lemon, straining out the seeds, and add both to the bowl of a food processor. Add the garlic, arugula, Pecorino, almonds, red pepper flakes, 1/2 teaspoon salt and pepper to taste. Pulse everything until it's finely chopped, scraping down the sides of the bowl.
With the food processor running, slowly pour in the olive oil until everything is blended into a paste. Taste and add more salt, pepper, lemon juice or olive oil if needed. Transfer the pesto to an airtight container and refrigerate it for up to 10 days.
Pesto is a make-ahead dream. It's a means for cleaning out your fridge and pantry, it freezes beautifully, and it plays nice with everything from meat and fish to pasta and grains to soups, salads and vegetables.