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- ¼ cup Hy-Vee unsalted butter, divided
- 1 (8-oz.) pkg. white sliced mushrooms
- 1 medium yellow onion, thinly sliced
- 4 Hy-Vee gourmet mushroom steakhouse burger patties
- 4 slices Hy-Vee Swiss cheese slices
- 1 medium red bell pepper
- 4 Hy-Vee wheat hamburger buns
Melt 2 tablespoons butter in a large skillet over medium-high heat. Add mushrooms and season with salt. Cook 3 to 5 minutes or until mushrooms have softened. Remove from skillet; cover and keep warm.
Add remaining 2 tablespoons butter to skillet; add onions. Season with salt and reduce heat to medium-low heat. Cover and cook 20 to 25 minutes, stirring occasionally, or until onions are very soft and golden brown. Remove skillet from heat; cover and keep warm.
Prepare a charcoal or gas grill with greased grill grates for direct cooking over medium-high heat (400°).
Place burgers on grill. Grill 10 to 13 minutes, flipping once halfway through; top with cheese and continue grilling 1 to 2 more minutes or until burgers reach 165°. During the last 5 minutes of grilling, add red bell pepper. Cook, turning occasionally until charred. Remove burgers and red bell pepper from grill; tent with foil and keep warm.
Meanwhile, place hamburger buns on grill grates. Grill 30 seconds to 1 minute or until lightly toasted. Remove from grill.
Top each bottom bun with even amounts of sautéed mushrooms and grilled red pepper slices. Top with cooked burger and even amounts of caramelized onions. Sandwich with top burger bun.