In a medium cast-iron or other heavy duty skillet, heat oil over medium-high heat. Add garlic and onion and cook for 1-2 minutes until onion is translucent.
Add in the chopped veggies: squash, bell pepper, and other peppers. Cook for 4-5 minutes until slightly tender but still crisp.
Remove from heat and stir in baby spinach until wilted. Add in fresh herbs now if using.
Using the same skillet (or a quesadilla maker), place one tortilla over medium heat. On one half of the tortilla add 1/4 cup of shredded cheese. Cover the cheese with 1/2 cup of the cooked veggies, and top with another 1/4 cup of cheese. Fold over tortilla and cook, flipping as necessary, until cheese is melted and tortilla is crispy.
Serve warm with salsa or guacamole.