Meanwhile, toast the pine nuts in a dry skillet over medium heat, watching carefully, so they don’t burn. Toasting the pine nuts will take about 6-7 minutes. Transfer the pine nuts to a plate. Set aside to cool.
When the water boils, add about 2 tablespoons of salt. Add pasta and cook according to package directions for al dente pasta.
Drain the pasta when done. Cool under running water. Shake off excess water, then pour pasta into a large bowl.
Add lemon juice to the pesto. Add pesto to pasta, and toss to combine
Add salt and pepper, spinach, tomatoes, and onion. Toss again.
Top with grated parmesan cheese and toasted pine nuts.
Notes
Storage: Leftovers may be stored safely in a covered container in the refrigerator for up to 5 days. It tastes best if eaten within about three days. Freezing is not recommended.