Heat 3 tablespoons of olive oil in a large pan. Add onion, stirring frequently until onion has reduced in size and browned. Caramelization will take about 20-25 minutes.
Meanwhile, bring another pot of water to a boil and cook pasta according to package directions.
Once onions have caramelized, remove from pot and set aside. Add another tablespoon of of oil to pot and immediately add garlic.
Cook garlic on very low heat until it begins to brown. Be careful not to burn. Then, add in bell pepper and increase heat to medium. Cook until slightly softened.
Add in zucchini and broccoli and cook for 5 to 7 more minutes.
Add in mushrooms and cook until they release their liquid and cook down. Add seasonings and stir.
Remove pot from heat and add in spinach until wilted. Stir in basil and additional seasoning to taste, as needed.
Serve hot and enjoy!