French Onion Soup Burgers

Fill a medium pot 3/4 of the way up with salted water; cover and heat to boiling on high. Wash and dry the fresh produce. Place the mushrooms in  a bowl; add 1 cup of hot water. Set aside to rehydrate at least 10 minutes. Medium dice the potatoes. Thinly slice the scallions. Halve, peel, and thinly slice the onion. Grate the cheese on the large side of a box grater. Halve the buns.

Add the diced potatoes  to the pot of boiling water. Cook 14 to 16 minutes, or  until tender when pierced with a fork. Turn off the heat. Reserving the pot, drain thoroughly and transfer to a large bowl. Add the creamy mustard sauce, pickle relish, sliced scallions, and a drizzle of olive oil. Gently stir to combine. Taste, then season with salt and pepper if desired. Place in the refrigerator to cool until ready to serve.

Meanwhile, in a large pan (nonstick, if you have one), heat a drizzle of olive oil on medium-high until hot. Add the sliced onion; season with salt and pepper. Cook, stirring occasionally, 8 to 10 minutes, or until browned and softened. Add the vinegar (carefully, as the liquid may splatter) and worcestershire sauce. Cook, stirring frequently, 30 seconds  to 1 minute, or until combined. Transfer to a bowl. Taste, then season with salt and pepper if desired. Cover with foil to keep warm. Rinse and wipe out the pan.

Place the beef in a bowl; season with salt and pepper. Gently mix to incorporate. Form the mixture into four 1/2-inch-thick patties. In the same pan, heat a drizzle of olive oil on medium-high until hot. Add the patties. Cook 4 to 5 minutes per side (flipping carefully, as the oil may splatter), or until browned and cooked through.* Transfer to a plate. Rinse and wipe out the pan.

*An instant-read thermometer should register 160°F.

Meanwhile, reserving the mushroom liquid, drain the rehydrated mushrooms; discard the mushrooms. In the same pot, heat 1 tablespoon of olive oil on medium-high until hot. Add the flour. Cook, stirring frequently with a wooden spoon, 1 to 2 minutes, or until lightly browned and thoroughly combined. Add the reserved mushroom liquid (carefully, as the liquid may splatter). Cook, whisking constantly, 3 to 4 minutes, or until the sauce is thickened. Turn off the heat; whisk in the grated cheese until melted and combined. Season with salt and pepper.

Add the halved buns, cut side down, to the same, dry pan. Toast on medium-high 30 seconds to 1 minute, or until lightly browned. Transfer to a work surface. Assemble the burgers using the toasted buns, cooked patties, cheese sauce, cooked onion, and crispy onions. Serve the burgers with the potato salad on the side. Enjoy!