While water heats, toast the pine nuts in a dry skillet over medium heat. This will take about 6-7 minutes while continuously tossing so it doesn’t burn. Transfer the pine nuts to a plate and set aside.
Add 2 tbsp of salt to boiling water then add pasta and cook according to package directions for al dente pasta.
Drain the pasta when finished and cool under running water. Shake off excess water, then pour pasta into a large bowl.
Add lemon juice to the pesto, then add pesto to pasta. Add in spinach, tomatoes, onion, salt and pepper. Toss well to combine evenly.