Follow these simple steps to prepare your keto cheeseburger stuffed portobellos:

1. Preheat oven to 375°F (190°C).
2. Prepare the mushrooms: Remove stems from portobello mushrooms and place them on a baking sheet.
3. Sauté the aromatics: In a skillet, sauté onions and garlic until soft.
4. Cook the meat: Add ground turkey or beef, Worcestershire sauce, mustard, salt, and pepper to the skillet. Cook until the meat is browned.
5. Stuff the mushrooms: Stuff each portobello with the meat mixture and top with shredded cheese.
6. Bake: Place the stuffed mushrooms in the oven and bake for 15-20 minutes or until the cheese is melted.
7. Serve: Serve on lettuce leaves as “buns.”