Heat oil in a large skillet over medium-high heat. Add onion, bell peppers, and mushrooms; sauté 5 minutes. Combine onion mixture and next 5 ingredients (through beef) in a bowl. Shape beef mixture into 8 (1/3-inch-thick) patties.
Heat a grill pan over high heat; coat with cooking spray. Cook patties 3 minutes; turn, top with cheese, and cook 2 minutes.
Arrange buns, cut sides up, on a baking sheet; broil 30 seconds. Assemble sandwiches.
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These are yummy and easy to prepare. We made 4 full-size burgers, and like the other reviewers I had a hard time keeping them together. Overall a little dry - a sauce or something to slather on the buns would have been nice.
These are pretty tasty. My only complaint is that they tended to fall apart on the grill. Next time I'll probably just make them into full-sized burgers. We had them with a small bag of pop chips and some grapes. Quick, easy, weeknight meal.