For the steak: To a large resealable bag, add the red wine vinegar, hot sauce, 1 teaspoon salt, Worcestershire sauce, paprika, 1/2 teaspoon black pepper and lime zest and juice. Mix together and add the flank steak. Marinate for 1 hour in the refrigerator.
Preheat the grill to medium-high.
Remove the steak from the marinade and discard the marinade. Pat the steak dry and sprinkle with salt and pepper. Grill over medium-high heat until cooked to medium-rare, 5 to 6 minutes per side. Let rest 10 minutes, loosely covered with aluminum foil, before slicing.
For the gravy: In a large pan with straight sides, add the butter, olive oil, onions and thyme. Season with salt and pepper and cook over medium heat until the onions are tender and translucent, about 5 minutes. Add the mushrooms and garlic, season with salt and cook gently until the mushrooms are tender and darkened, about 5 minutes more. Add the Worcestershire sauce and horseradish. Sprinkle the flour over the entire pan. Stir to help the flour to soak into the vegetables and cook a minute more to lose the flour flavor. Raise the heat to medium-high and add the beef stock. Cook until slightly thickened, then slowly stir in the heavy cream. Simmer until the cream is thickened slightly, about 4 minutes more. Taste and season with salt, if needed.
For the toast: Preheat the oven to 350 degrees F or a grill to medium heat.
Arrange the sliced bread on a baking sheet lined with aluminum foil. In a small pot, add the butter, onion powder, thyme and garlic and season with salt. Heat over medium-low heat until the butter is melted. Brush the garlic butter evenly onto the bread. If baking, place in the oven and bake until the edges are golden, about 10 minutes. If grilling, place the bread slices buttered-side down on the grill and grill until toasted and grill marks appear, 2 to 3 minutes.
To serve, layer the toast, then slices of beef and pour over a spoonful of gravy.