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.