Pork Patty Melts Recipe

Name: Pork Patty Melts Recipe
Ingredients: 1 pound ground pork (see note); 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