To prepare the tomato sauce, heat the oil in a frying pan and sauté the onion until softened, this will take about 5 minutes and then add the garlic and cook for 1 minute. Stir in the tomatoes, tomato paste and the chilli. Add the sugar and season with salt and freshly ground black pepper.
Simmer slowly for about 15 to 18 minutes. Preheat the oven to 200°C/ fan 180°C / gas mark 5. Brush an ovenproof dish with melted butter. Place each tortilla, flat on a surface, sprinkle with some cheese, chopped onion, shredded chicken, chilli, 2 tbsp. of tomato sauce, a spoon of sour cream and lime zest.
Retain some cheese and sauce and sour cream for the top. Roll up all the tortilla and place them seam-side down in the baking dish next to each other. Pour over the remaining tomato sauce, sour cream and sprinkle the rest of the grated cheese on top. Bake in the preheated oven for about 15 to 20 minutes until heated through and just lightly golden on top.
To make the salsa, combine the avocado, coriander leaves, salt and lime juice and zest into a small serving bowl to serve with the enchiladas.