Grilled Pastrami, Swiss, and Sweet Onion Marmalade on Rye

INGREDIENTS

• 1 tablespoon butter
• 1 tablespoon vegetable oil
• 2 medium red onions, thinly sliced
• 1 large garlic clove, minced
• 1/2 cup sugar
• 1/2 teaspoon salt
• 1 teaspoon freshly ground black pepper
• 1/2 cup red-wine vinegar
• 1/2 cup dry but fruity red wine, such as Zinfandel or Grenache
• 4 slices extra-sour rye or caraway rye bread
• 2 tablespoons softened unsalted butter
• 4 thin slices pastrami, uncured if available
• 4 ounces coarsely shredded Swiss cheese
• 2 tablespoons whole-grain mustard