In a small bowl, mash the avocado, stir in the lemon and salsa, and season with salt and pepper. Heat a skillet and melt about 2 tsp of butter. While skillet is heating, build your sandwich in this order: slice of toast, slice of cheese, avocado mixture, tomatoes, salt and pepper, the final slice of cheese and the last slice of toast. Put the whole thing in the skillet, and heat each side until the cheese is melted. Don’t be afraid to press down slightly with spatula while grilling. You can also do this in a panini press.