Step 1

If you can’t find knockwurst, substitute high-quality beef hot dogs. We like to serve these with pickle spears and potato chips.

Step 2

Adjust oven rack to middle position, place rimmed baking sheet on rack, and heat oven to 200 degrees. Spread mustard on 1 side of each bread slice. Divide pastrami into 4 piles on cutting board, each about size of 1 slice of bread. Split each knockwurst lengthwise, stopping short of cutting completely in half, so sausages are hinged on 1 side.

Step 3

Melt 1 tablespoon butter in 12-inch nonstick skillet over medium heat. Arrange pastrami piles in skillet and cook until lightly browned on bottom, about 2 minutes. Flip pastrami and divide Swiss cheese among piles. Cover and cook until cheese is melted, about 2 minutes. Transfer pastrami to 4 slices of bread. Melt 1 tablespoon butter in now-empty skillet over medium heat. Cook knockwurst, covered, until browned and heated through, about 2 minutes per side. Arrange knockwurst over hot pastrami. Top with remaining bread, mustard side down.

Step 4

Melt 2 tablespoons butter in now-empty skillet over medium heat. Place 2 sandwiches in skillet and cook, covered, until golden on first side, 1 to 3 minutes. Flip sandwiches and continue to cook, covered, until second side is golden, 1 to 3 minutes longer. Transfer sandwiches to sheet in oven to keep warm. Repeat with remaining 2 tablespoons butter and 2 sandwiches. Serve.