Pesto Cream Sauce

Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain. Heat olive oil in a large skillet over medium heat. Saute onion until tender and translucent. Stir in garlic and butter, and saute until garlic is soft and fragrant, about 1 minute. Dissolve flour in milk, then stir in. Season with salt and pepper, and simmer 4 minutes, stirring constantly. Add cheese, and stir until melted. Stir in pesto. Add shrimp, mushrooms and tomatoes. Cook 4 minutes, or until heated through. Toss with pasta until evenly coated.