Step 1

Preheat oven to 400°F. Lay a piece of heavy-duty foil on a baking sheet and spray with nonstick spray.

Step 2

Place mushroom caps on the center of the foil, rounded sides down. Season with 1/8 tsp. each garlic powder and onion powder. Top with another large piece of foil. Fold together and seal all four edges of the two foil pieces, forming a well-sealed packet.

Step 3

Bake for 20 minutes, or until tender. Cut packet to release steam. Remove foil, and carefully drain excess liquid. Thoroughly blot dry.

Step 4

Meanwhile, bring a skillet sprayed with nonstick spray to medium-high heat. Add beef, onion, salt, pepper, and remaining 1/8 tsp. each garlic powder and onion powder. Cook, stir, and crumble until beef is fully cooked and onion has softened, 5–7 minutes.

Step 5

Remove skillet from heat. Stir in tomato, pickles, ketchup, and mustard. Divide between mushroom caps, and top with cheese. Microwave for 30 seconds, or until melted.