Preheat the oven to 350°F (180)°C. Cook the rice according to the package instructions in a small pot. Usually by bringing 1 cup of rice with 2 cups of water to a boil. Reduce the heat to a simmer, cover the pot, and keep it on low heat until all of the water has been absorbed.
Meanwhile, add the oil to a large skillet over medium-high heat. Once heated, cook the mushroom slices for 3-4 minutes as they start to sweat. Then, add the onions and sauté them until they soften.
Lower the heat to medium, then add the flour to the pan. Stir until the vegetables are coated. Slowly add the broth and continue stirring for 3-4 minutes or until the clumps have smoothed out.
Stir in the plant milk, garlic powder, salt, and pepper. Cook this for several minutes as the mixture thickens. Then, mix in most of the vegan cheese (save some for toppings).
Remove the pan from the heat, then stir in the sausage pieces, frozen peas, frozen corn, and cooked rice.
Spoon everything into a medium-sized casserole dish. Top it with the remaining vegan cheese. Cover the dish with an oven-safe lid or foil and bake for 20 minutes.