Grill burgers over high heat until done, turning once (about 5 min. per side for medium doneness). Remove from heat; let stand, loosely covered with foil, for 3-5 min. Spread mayonnaise evenly on 1 side of each slice of bread. Place ½ of the bread slices, mayo side down, on a nonstick skillet or griddle preheated to medium. Evenly top bread with Cheddar, burgers, onions and Swiss cheese; top with remaining slices of bread, mayo side up. Cook sandwiches on first side until the bottom is dark golden-brown (2-3 min.); flip and repeat. If needed, reduce heat slightly and continue cooking, flipping occasionally, until cheese is melted and hot. Cut each sandwich in half on the diagonal; serve immediately.
1 ⅓ lbs. Kowalski's Prime or Akaushi Ground Chuck, formed into 4 patties
3 tbsp. mayonnaise
8 slices seeded rye bread
4 slices Kowalski's Sliced Cheddar Cheese
4 slices Kowalski's Sliced Swiss Cheese