Heat the butter in a medium skillet set over medium-high heat. Sprinkle the steaks with salt and pepper and sear on both sides, about 2 minutes per side for medium rare. Set aside to rest for 5 to 10 minutes.
Heat the same skillet over medium heat and add more butter if needed. Add the mushrooms, onions and garlic and cook until the onions are translucent and the mushrooms "weep," or release water, about 5 minutes. Then add the flour, 1 tablespoon salt, 1/2 teaspoon pepper and the mustard, and stir to combine. Add the, milk, broth, beer and Worcestershire. Mix in the pan to combine. Bring to a boil, then reduce to a simmer and cook until thickened, 4 to 5 minutes. Stir in the parsley and season with salt and pepper.
Preheat the oven to 425 degrees F.
Put the toast on a baking sheet and toast until golden brown, 5 to 10 minutes.
Place a piece of toast on each plate. Slice the beef against the grain and place some on top of the toast. Spoon the sauce on top and garnish with some parsley.