In a large skillet, melt butter. Add chopped onion and cook until onion begins to turn transparent and edges become golden brown. Add ground beef and salt and cook until meat is no longer pink. Drain off any grease.
Add tomato sauce and simmer for 10 minutes.
In another large skillet melt some butter (about 1 tablespoon to start) over medium heat (about a tablespoon) and place a tortilla in skillet. Sprinkle with about ½ cup (or enough to cover tortilla) grated cheese and then some meat mixture (about ¾ cup). Top with another tortilla and spread butter all over top of tortilla. Cook until golden brown and then flip and cook other side until golden brown. Repeat with remaining tortillas, adding more butter to the pan as needed.
Remove from pan and slice into triangles.
Top with sour cream, fresh cilantro and guacamole, if desired.