A far cry from the standard grilled-mushroom-cap-as-burger, these ultra-savory portobello patties get a further umami boost from garlic and steak sauce — a perfect match for a flavorful topping of melted blue cheese and tangy caramelized red onions.
For the Portobello Burgers
- 8 large portobello mushroom caps
- 4 cloves garlic, minced
- 3 tbsp. steak sauce (such as A1)
- 3⁄4 cup breadcrumbs
- 1 egg
- 1⁄2 tsp. fresh-ground black pepper
- 2 tbsp. olive oil
- 4 oz. blue cheese, crumbled, for serving
- 4 sesame seeded hamburger buns, split and toasted, for serving
For the Onions
- 3 tbsp. butter
- 1 large red onion, thinly sliced
- 2 tbsp. balsamic vinegar
- 2 tsp. dried thyme
- Salt and pepper, to taste
Make the portobello burgers: place mushroom caps in a bamboo steamer over simmering water, or over a steamer insert in a 6 quart saucepan. Cover and steam until mushrooms are tender, about 7 minutes. Place cooked mushrooms in a food processor with garlic, steak sauce, bread crumbs, and egg, and season with black pepper. Pulse just until mixture comes together, about 20 pulses or 30 seconds. Divide mixture into four 6 oz. patties, 4-inches wide by 1⁄3-inch thick. Place on a plate and refrigerate for 20 minutes or until ready to use. (See note.)
Make the onions: Heat butter in a 12-inch skillet over medium-high heat. Sauté onion until soft, about 8 minutes. Add vinegar, thyme, salt, and pepper and cook until onions are slightly caramelized, about 7 minutes more. Remove from the pan and reserve.
Wipe out skillet and heat olive oil over medium-high. Cook portobello burgers, until toasted on each side and cooked through, about 12 minutes (6 minutes per side). Top each burger with 1 oz. cheese and continue to cook until just melted, about 2 minutes. Divide cheese-topped burgers between toasted buns, top with onions, and serve.
Note: Individual portobello burgers can be formed and then frozen, sealed, for up to one month. Thaw in the refrigerator and cook as needed