Shape beef into 6 patties.
Heat a large skillet over medium – high heat, add butter and melt until it foams.
Add spring onion and garlic, sauté until softened, about 30 seconds.
Season beef patties with salt and pepper, and cook to the desired degree of doneness, turn once, set aside and keep warm.
In the same skillet, add and sauté the mushrooms until limp, then add to the warm beef patties.
To prepare the sauce, deglaze the pan with dry white wine. Reduce volume by half. Add Dijon mustard, heavy cream, tomato, and capers. Heat slowly until hot but do not boil.
Pour mustard cream sauce over beef and mushrooms