In a large skillet, melt the butter and fry the 2 eggs - over medium or over well. Fry for about 3 minutes, then reduce heat to medium, flip and fry another 2-3 minutes.
Mash the avocado and mix with lemon juice and salt and pepper to taste.
Assemble the toast with tostada shell, avocado spread, fried egg and dash of hot sauce or salsa on top.