Recipe courtesy of Stephanie Surowiec, finalist of the 2020 Blended Burger Project Home Edition Competition.
Make the blended burger patties: Melt 1 tablespoon butter and 1 tablespoon olive oil in sauté pan. Sauté the shallots and garlic with thyme over medium-low heat so as not to burn them. Add in chopped mushrooms, along with 1 tablespoon butter and 1 tablespoon olive oil, and sauté until fully cooked and all liquid has evaporated. Add sherry wine to pan and again cook until liquid is gone. Season with salt and pepper, remove thyme sprig, and allow to cool completely. You should end up with about 8 ounces of cooked mushrooms. Combine with 16 ounces ground chuck and form into 3 ounce balls. Each smashburger will be comprised of two-3 ounce patties.
Make the balsamic onions: Melt 1 tablespoon butter and 1 tablespoon olive oil in sauté pan. Thinly slice the Vidalia onion and cook over medium-low heat about 20 minutes, caramelizing the onions. Add in brown sugar, then balsamic vinegar and Worcestershire sauce, and cook until liquid reduces, creating jammy onions. Season with salt and pepper.
Make the Cornichon Aioli: Combine 1/2 cup homemade aioli, plus 2 tablespoons finely minced cornichons. Alternatively, use 1/2 cup high quality store-bought mayo, 1 teaspoon Dijon mustard, 1 tablespoon lemon juice, 1 clove finely minced garlic, pinch of salt, and 2 tablespoons finely minced cornichons.
Grill each burger patty on an oiled flat top griddle for about 3 minutes per side, or until edges are charred and crispy. Flip the patties over and top both with Gruyere cheese. Layer patties on top of each other, and transfer to bottom half of toasted brioche buns. Top patties with balsamic onion. Add a good schmear and cornichon aioli to the top half of the toasted brioche buns. Top with toothpick and extra cornichon.