1. Heat 1 tablespoon each of the butter and olive oil in a large, heavy skillet over medium-low flame. Add the sliced onions and cook, stirring often, until onions are very limp and deeply colored, 15–20 minutes. Meanwhile, cut away and discard tough portions of mushroom stems. Wipe the mushrooms clean with paper towels and slice them.
2. When onions are done, remove them to a small bowl. Wipe out the skillet, add the remaining butter and olive oil and raise heat to medium-high. When butter foam subsides, add sliced mushrooms and cook, stirring occasionally, until mushrooms are tender, about 10 minutes. Stir in the rosemary and some salt and pepper about halfway through the cooking. If excess liquid accumulates in the pan, raise the heat to high to reduce the liquid.
3. Stir in mustard and heavy cream; cook, stirring often, until cream is lightly thickened and “saucy.’ Add the reserved onions and season to taste with salt and pepper.
4. While mushroom sauce is cooking, season the ground beef generously with salt and pepper. Form into six large, 1-inch-thick patties (but don’t overwork the mixture). Heat a large, cast iron skillet (or two medium skillets) over medium-high flame for several minutes. Pan-grill the burgers to desired doneness, 4–6 minutes per side. Meanwhile, split the buns in half and lightly toast them. Transfer burgers to buns and top with mushroom sauce. Serve immediately, with additional mustard on the buns, if desired.
Copyright by Terese Allen