Pork Patty Melts

Ingredients

• 1 pound ground pork
• kosher salt and freshly ground black pepper
• 2 teaspoons olive or vegetable oil
• 8 tablespoons (1 stick) unsalted butter
• 1 large yellow onion, split in half and thinly sliced from pole to pole (about 2 cups)
• 1 fennel bulb, trimmed, cored, and thinly sliced (about 2 cups)
• 1 1/2 teaspoons red pepper flakes
• 3 garlic cloves, thinly sliced
• 2 tablespoons malt vinegar
• 8 slices sturdy white sandwich bread
• 12 slices swiss cheese, torn in half