Cut the avocadoes in half, remove pits, and scoop the flesh into a bowl. Add the lemon juice and sea salt, to taste. Mash the ingredients together with a fork, keeping the texture slightly chunky. Mix in the tomato, onion, and cilantro.
Put a cast iron skillet over medium high heat. When the pan is hot, put in a corn tortilla and cook until the tortilla is heated through and softened, about 30 seconds per side.
Spread some of the guacamole onto a heated tortilla, fold it in half, and eat immediately. Repeat with remaining ingredients.