In 10-inch nonstick skillet, cook ground beef and onion over medium-high heat, stirring frequently, until beef is thoroughly cooked; drain. Stir in tomato paste, Italian seasoning and mushrooms. Cover and refrigerate; use within 3 days.
To prepare each sandwich, spread about 1/4 cup beef mixture on English muffin half. Sprinkle 2 tablespoons cheese over top. Place on microwavable napkin or paper towel (to absorb moisture).
Microwave 1 sandwich uncovered on High 1 to 2 minutes or until beef mixture is hot and cheese is melted.