Heat the oil in a medium non-stick skillet over medium-high heat.

Reduce the heat to medium-low, add the onion, cover and cook, stirring occasionally, until the onions are golden, about 10 minutes.

Remove the lid and cook, uncovered, stirring frequently until the onions are golden brown and the edges are browned, 10-12 additional minutes.

Stir in the curry powder and cayenne pepper.

Onions may be made up to 3 days ahead and stored in the refrigerator.

Place one slice of cheddar cheese on each of 4 slices of the bread.

Spread a quarter (about 2 Tbs ) of the caramelized onions on top, then place one large or 2 medium slices tomato.

Place another slice of cheese on top of that, then the other slice of bread.

Wipe out the skillet, then coat with cooking spray and heat over medium-high heat until hot.

Place one of the sandwiches in the skillet and weigh down with a smaller heavy skillet .

Lower the heat to medium-low and grill until the underside is a deep brown and the cheese is partially melted, 3-4 minutes.

Flip the sandwich and grill the other side, another 3 minutes.