Preheat the oven to 375 degrees F (190 degrees C).
Heat a large skillet over medium-high heat; grease with olive oil spray. Add mushrooms and onion; saute until soft, about 5 minutes. Add diced tomatoes, gravy, and frozen vegetables. Bring to a boil. Reduce heat to low and cook until flavors blend, 8 to 10 minutes.
Melt vegan margarine in a saucepan over medium heat. Add water and garlic powder; bring to a boil. Remove from heat. Add rice milk and stir in mashed potatoes and egg substitute.
Spoon mushroom mixture into an ungreased 8-inch glass baking dish. Spread potato mixture on top.
Bake in the preheated oven until top is golden brown, about 20 minutes.