Open-Face Steak and Onion Sandwiches

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3 tablespoons unsalted butter, at room temperature

2 onions, thinly sliced

Kosher salt

1 tablespoon plus 2 teaspoons Worcestershire sauce

3/4 cup shredded white cheddar cheese

3 tablespoons ketchup

2 tablespoons yellow mustard

1 tablespoon steak sauce

1 tablespoon mayonnaise

Freshly ground pepper

1 1/2 pounds skirt steak, cut into 4 pieces

4 thick slices crusty sourdough bread

1 bunch watercress, trimmed