Portobello Mushroom Burgers with Caramelized Onions

Ingredients

  • 1 ounce unsalted butter
  • 3 pounds red onions, peeled and sliced
  • 4 aidells portobello mushroom sausage (cut into 1/2-inch chunks)
  • 4 slices italian fontina cheese
  • 4 crusty rolls, sliced
  • 4 tablespoons coarse grain mustard