Heat a skillet or pan over low heat, then lay two of the bread slices (butter side down) into the pan. On each bread slice, place 2 slices of cheddar cheese and 2 tomato slices, then top with jalapeño and onion.
Place the two other bread slices on top of the sandwiches, butter side up.
Continue to grill the sandwich until the bottom side is a golden brown, then flip the sandwich. Continue to grill until the other side is a golden brown as well.
Remove the sandwiches from the grill, cut in half, and serve.
Want it spicier? Try serrano peppers instead of jalapeños. Serranos are typically double the spiciness, so approach with care.