In a food processor, add arugula, walnuts, cheese, and garlic. Pulse until combined. Add olive oil a little at a time until you have the consistency that you'd like. If you want to spread this on toast you will want a thicker pesto. If you want to use it in pasta, you may want a thinner pesto, so then add more oil. Once you have the consistency you'd like, add salt and pepper to taste.