- Preheat oven to 350 degrees. Line a baking sheet with parchment paper. 
- Thoroughly mix crust ingredients in a large bowl. Shape into a circle on the parchment-lined baking sheet, about 1/4-inch thick and 10 inches in diameter. 
- Bake until cooked through, about 15 minutes. 
- Meanwhile, bring a skillet sprayed with nonstick spray to medium heat. Add mushrooms, onion, and bell pepper. Cook and stir until mostly softened and lightly browned, about 4 minutes. 
- In a medium bowl, combine crushed tomatoes, Italian seasoning, onion powder, and garlic powder. Mix well. 
- Remove crust from oven, but leave oven on. Carefully drain excess liquid from the baking sheet, and thoroughly blot crust dry. 
- Spread seasoned tomatoes over crust, leaving a 1/2-inch border. Top with cheese and cooked veggies. 
- Bake until cheese has melted, about 5 minutes.