Prepare a grill or grill pan for medium-high heat. Cut a 30-by-12-inch piece of aluminum foil.
Mix together the sour cream, mustard and a few grinds of pepper in a small bowl. Cover and set aside.
Rub the flank steak all over with the Worcestershire sauce, 1 teaspoon salt and 1/4 teaspoon pepper. Drizzle the mushroom caps with 1 tablespoon of the oil.
Place the onions in a single layer in the middle of the foil. Drizzle with the remaining tablespoon oil, and sprinkle with 1/2 teaspoon salt and a few grinds of pepper. Fold over the foil; crimp all the open sides tightly while keeping the packet flat.
Lightly oil the grill grates. Grill the steak 7 to 8 minutes per side for medium-rare. Let it rest 10 minutes, covered loosely with foil, then slice thinly across the grain.
Meanwhile, place the onion foil packet and the mushrooms on the grill. Cook the foil packet for 4 minutes per side; remove from the grill, and keep sealed for 5 minutes so the onions continue to soften as they cool. Grill the mushrooms until tender, about 3 minutes per side; slice thinly, and season with 1/4 teaspoon salt and some pepper.
Grill the bread slices until you see grill marks and some char, 30 seconds to 1 minute per side. Rub each grilled slice with the cut side of a garlic clove. Drizzle with oil.
Assemble the open-faced sandwiches: On each slice of bread, spread 1 tablespoon of the sour cream sauce. Top with the steak, mushrooms, onions, another tablespoon of sauce and parsley. Finish with a squeeze of lemon juice and some salt and pepper.