Melt butter in a saucepan over medium heat. After butter has melted, stir in flour until combined and then add milk. Stir until sauce has thickened, about 20 minutes. Season with salt and pepper to taste.
Heat olive oil in a large nonstick skillet over medium high heat. Add garlic and onion and sauté for 3 to 4 minutes. Add frozen peas and carrots and sauté until cooked.
Combine cooked pasta, white sauce, and sautéed vegetables and serve.