In a small bowl combine the tomato ketchup, harissa and seasoning to taste. Mix well and spread on the base of one of the tortillas.
Scatter the peppers and 1/2 of the onion on top of the sauce. Finally scatter over the cheese slices to cover. Place the other tortilla on top.
In a frying pan, using a pastry brush, lightly oil the pan with olive oil. Place the tortilla in the pan. Brush the top of the tortilla with olive oil again with a pastry brush.
Pan fry gently over medium to low heat until the tortillas have browned slightly on both sides and the cheese has melted well. This will take approx 8 to 10 minutes. Carefully flip over the tortilla a few times whilst cooking.
Once cooked, remove from the pan and keep warm. Add a little more oil to the pan and fry off the remaining onion and fry the egg. The egg is best cooked with the yolk still runny.
Cut the tortilla into 4 wedges. Arrange on a plate and top off with the fried onion and egg. Scatter over the basil and serve.