Bring a large pot of water to a boil, add spaghetti and cook until pasta is al dente.
In a skillet, heat olive oil in medium to high heat.
Once hot, add garlic and onion, sauté until golden. Add broccoli and cook until tender. Add the peppers, mushrooms and seasonings. Once mixture is tender, remove skillet from heat. Combine vegetables and pasta.
In a bowl, mix lemon juice with Greek yogurt and dijon mustard.
Pour yogurt sauce over pasta and stir until evenly coated.