• In a large skillet over low heat, melt 1/2 tablespoon butter. Add the onion, 1/2 teaspoon salt, and pepper to taste. Cook on low heat, stirring occasionally, until soft and golden, or about 20-25 minutes. Continue cooking, stirring occasionally, until the onions are extremely tender and the liquid is slightly syrupy, or about 8 more minutes. Remove from heat and stir in Worcestershire sauce and mustard. Transfer the onions to a small bowl and set aside.
• In the same pan, add the pastrami and lightly crisp the meat until just warmed through. Remove from heat and set aside.
• Lightly butter the outsides of all 4 pieces of bread. On the inside of the bread, divide half of the Cooper® cheese between 2 of the bread slices. Top each piece of bread with equal amounts of hot pastrami and onions. Top with the remaining cheese, then place the remaining 2 slices of bread on top of the Cooper® cheese, with buttered sides facing out.
• Melt the remaining 1 tablespoon butter in a large skillet over medium heat. Add the sandwiches and cook, pressing occasionally with a spatula, until the bread is toasted and the cheese melts, or about 4 minutes per side. While cooking, add more butter to the pan if needed.
*Consuming raw or undercooked meats, poultry, seafood, shellfish, or eggs may increase your risk of foodborne illness.