2 tsp canola oil
1 red onion, finely diced
3 oz sharp Cheddar, thinly sliced, divided
8 slices of whole-wheat bread (although sourdough really, really makes a grilled cheese)
3 oz pepper-jack cheese, thinly sliced, divided
1 large or 2 medium tomatoes, thinly sliced
Heat oil in a nonstick skillet over medium-high heat. Lower heat to medium; add onions and saute, stirring, until edges are brown. Season with salt and pepper to taste.
Place 3/4 oz. Cheddar on one slice of bread. Spread 1 TBSP carmelized onions on top of cheese, then top with 3/4 oz. pepperjack. Top with 1 or 2 slices of tomato, then another slice of bread. Repeat with 3 other sandwiches.
Spray a nonstick skillet with cooking spray and heat until hot. Flip when browned on the bottom (you knew that already).