To prepare a vegetarian mexican breakfast casserole recipe, first you will need to prepare the ingredients for the casserole. First, heat the oven to 375 degrees F (190 degrees C). Then, add the flour, baking powder, salt, and pepper to the dry ingredients and mix until they are evenly combined. Next, stir in the milk and stir until it is completely incorporated. Finally, pour the mixture into the prepared casserole dish and bake for 35 to 40 minutes, or until the top is brown and bubbly.