Preheat oven to 425°.
Place crust on a baking sheet; set aside.
Combine cornstarch, milk, and garlic in a small saucepan, stirring well with a whisk. Bring to a boil over medium heat; cook 1 minute, stirring constantly. Stir in salt and pepper. Spread sauce over crust.
Heat a large nonstick skillet coated with cooking spray over medium-high heat; sauté shrimp 3 minutes. Add mushrooms; sauté 2 minutes. Stir in spinach; cook 1 minute or until spinach wilts. Drain. Spoon shrimp mixture over sauce. Sprinkle with oregano and basil. Top with cheeses. Bake at 425° for 8 minutes or until cheese is melted.