Steak Sandwich With Romesco, Grilled Onions, and Havarti

Preheat grill to medium-high. Brush the grill grates with oil. Brush the onion with 1 tablespoon of the oil and sprinkle with ½ teaspoon of the salt and ¼ teaspoon of the pepper. Brush the steak with the remaining oil and season with the remaining salt and pepper.

Grill the onion, covered, until tender and charred, 6 to 8 minutes per side. Grill the steak, 3 to 5 minutes per side for medium-rare. Remove the onion and steak from grill. Let the steak stand for 5 minutes, then slice across the grain.

Brush one side of each bread slice with the butter. Top 4 of the bread slices with 1 cheese slice each. Grill all of the bread, covered, until the bread is toasted and the cheese is melted, 1 to 2 minutes. Divide the steak among the 4 pieces of plain toast; add 1 slice of onion and 1 tablespoon Romesco Sauce to each. Top with the remaining toast, cheese-side down, and serve.