In a large pot with boiling salted water cook spaghetti pasta until al dente.
Drain.
Meanwhile, in a large skillet heat olive oil over medium heat.
Add onion, garlic, basil, sliced mushrooms, and chopped tomatoes and cook for 5 minutes. Stir in California-style vegetables, salt, and ground black pepper. Cook for approximately 10 minutes, stirring often, until vegetables are tender and crisp.
Pour vegetable mixture over cooked and drained pasta. Toss well.