Marinated Flank Steak Sandwiches

Name: Marinated Flank Steak Sandwiches
Ingredients: 1 small white onion, sliced into ¼-inch rings; ½ cup fresh orange juice (from 2 oranges); ¼ cup fresh lime juice (from 2 to 4 limes); ¼ teaspoon ground cumin; 2 garlic cloves, crushed; 1 ½ pounds flank steak, trimmed of excess fat; Coarse salt and freshly ground pepper, to taste; 2 tablespoons unsalted butter, room temperature; 1 loaf ciabatta (16 inches), split horizontally, halved crosswise; 6 ounces Gruyere cheese, thinly sliced; 1 whole dill pickle, thinly sliced crosswise