Tomato Avocado Grilled Cheese Sandwich

4 slices sourdough bread
1 ripe avocado, cut in half, halves sliced
1 large ripe tomato, sliced
2 tablespoons thinly sliced red onion
Grated sharp or smoked cheddar cheese, about 1 cup packed
1/4 cup finely grated Parmesan cheese
Butter, softened

1. Spread one side of each of the bread slices with butter. Sprinkle the Parmesan cheese on a small plate and dredge the buttered bread sides in the cheese to fully coat the butter.
2. Start a panini press heating or heat a large cast iron pan over medium heat. Layer half the cheddar cheese on the unbuttered side of each slice of bread. Add the slices of avocado, tomato slices, and onion. Cover with the remaining cheese and top with bread slice, buttered/cheesed side up.
3. Cook in a panini grill until browned and cheese is melted. Or if cooking on a cast iron pan, place sandwiches on the pan and gently press with a metal spatula while they cook. Flip gently when the underside is browned. Press again to cook evenly and melt the cheese. Serve immediately.