Preheat oven to 400°F. Coat a 12-inch pizza pan with cooking spray and place it on a large baking sheet with sides.
Heat oil in a large skillet over medium heat. Add onion, bell pepper, mushrooms and basil; cook, stirring, until softened, about 10 minutes.
Meanwhile, combine beef, garlic, breadcrumbs, milk and salt in a large bowl. Mix well.
Transfer the meat mixture to the prepared pan. With dampened hands, pat into a 10-inch circle. Top with marinara sauce. Spoon the vegetable mixture over the sauce and sprinkle with cheese.
Bake until the meat is browned and the cheese has melted, about 30 minutes. Drain off any fat. Cut into wedges and serve.