Preheat the grill to high. Thinly slice the onion. Crush or finely chop the garlic. Remove the top of the chilli (the chilli is mild, but if you prefer less heat, use half or omit it). Combine the Mexican seasoning,cumin and coriander spice blend and 2 tsp olive oil in a large bowl and season with salt and pepper. Add the chicken and turn to coat.
Step 2 — Start chilli sauce
Heat 1 tbs olive oil in a small saucepan over high heat. Cook the onion and garlic, stirring regularly, for 3 mins or until the onion is starting to soften. Add the tomatoes and ½ tsp sugar and bring to the boil. Season with salt and pepper. Reduce the heat to medium and cook for 6 mins to develop the flavour.
Step 3 — Finish chilli sauce
Meanwhile, put the chilli on an oven tray and grill, turning halfway through cooking, for 6 mins or until blackened. Halve the chilli lengthwise and remove the seeds, then add to the sauce. Blend the sauce with a stick blender until smooth. Taste, then season with salt. Turn the grill to oven mode and preheat to 200C, fan-forced.
Step 4 — Cook chicken
Heat a medium frypan over medium-high heat. Cook the chicken for 3-4 mins each side until golden and cooked through. Remove from the pan and cool slightly, then coarsely shred or thinly slice. Combine the shredded chicken and 2 tbs chilli sauce in a bowl.
Step 5 — Bake enchiladas
Line a medium baking dish with baking paper. Lay the tortillas flat on a board. Spoon the chicken mixture down the centre of each tortilla and roll up to enclose, then put in the lined dish, seam-side down. Pour the remaining chilli sauce over the enchiladas, then scatter with the cheese. Bake for 18 mins or until golden and the cheese is melted.
Step 6 — Chop avocado and serve up
Coarsely chop the avocado and season with salt and pepper. Top the enchiladas with the chopped avocado and serve at the table for everyone to help themselves.