Cut the spaghetti squash in half lengthwise and scoop out the seeds. Cut into large chunks and steam in a steamer basket or bake face down on parchment paper at 400°F for 25-30 minutes, or until it is soft and shreddable with a fork.
Shred and fluff the spaghetti squash with two forks.
Make the pesto by processing everything in a food processor except the cheese and a few nuts, reserved for texture. Adjust with salt and lemon to taste, and stir in the cheese and reserved nuts.
Portion out your desired amount of spaghetti squash. Top with pesto and tomatoes.