Preheat your oven to 375°F (190°C) so it’s fully heated when the casserole goes in. Lightly spray a baking dish with nonstick cooking spray, making sure to coat the bottom and sides to prevent sticking and make serving easier later.

Heat a large skillet over medium heat and add the lean beef. Cook it slowly, breaking it up with a spoon as it browns. Continue cooking until the beef is fully cooked and no longer pink. If there is any excess liquid or fat in the pan, carefully drain it off to keep the casserole from becoming greasy.

Add a small amount of oil to the skillet, then add the sliced onion, bell pepper, and mushrooms. Sauté the vegetables over medium heat, stirring occasionally, until they soften and release their moisture. They should be tender and lightly golden, not watery.

Season the beef and vegetable mixture with salt, pepper, and any additional seasoning you prefer. Stir everything together well so the flavors are evenly distributed and the mixture looks cohesive.

Transfer the beef and vegetable mixture into the prepared baking dish and spread it out evenly. Sprinkle the reduced-fat cheese evenly over the top, making sure it covers the surface without piling up too thickly.

Place the baking dish in the preheated oven and bake for 20–25 minutes, or until the cheese is fully melted and lightly golden around the edges. Remove from the oven and let it rest for a few minutes before serving so the casserole sets slightly and is easier to portion.