Preheat an oven to 375 degrees F (190 degrees C). Grease a 9x5-inch loaf pan.
Combine flour, baking powder, salt, and garlic powder together in a small bowl. Set aside. Whisk together butter and eggs until well combined, then stir in the flour mixture. Fold in the onion, green bell pepper, red bell pepper, Cheddar cheese, and mushrooms. Mix well, the batter will be stiff. Pour batter into the prepared loaf pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 50 minutes to 1 hour. Cool in the pan for 10 minutes before removing to cool completely on a wire rack.