Bison Burgers with Cabernet Onions and Wisconsin Cheddar

INGREDIENTS

• 1 small head of escarole, leaves separated
• Dijon mustard
• 2 tablespoons olive oil, divided
• 1 pound ground bison (buffalo)
• 1/4 teaspoon dried thyme
• 3 cups sliced onions (about 2)
• 6 ounces sliced Wisconsin white cheddar cheese
• 3/4 cup Cabernet Sauvignon or other dry red wine
• 4 organic hamburger buns
• 2 tablespoons chopped shallots
• 1/4 teaspoon coarse kosher salt