1. Pre-heat the grill to full temperature.
2. Place the green peppers under the grill, turning every couple of minutes for 10 minutes until black all over.
3. Transfer to a bowl, cover with clingfilm and set aside for 15 minutes.
4. Peel the charred skin from the peppers, discard the stems and seeds and then slice the peppers.
5. In a frying pan warm the oil over medium-high heat. Add the onions and a pinch of salt and sauté, stirring until brown, for about 10 minutes. Stir in the green peppers.
6. In a small saucepan stir together the refried beans and half the salsa with the Major Fajita Mari Base. Stir until warm.
7. To assemble the sandwiches spread the bean mixture evenly over the bottom half of the rolls. Top with the pepper and onion mixture and then sprinkle evenly with the cheese. Dollop a little sour cream over each sandwich along with the rest of the salsa, some coriander and the shredded lettuce. Spread avocado slices evenly on the top half of the rolls. Close the sandwiches, pressing down to compress.
8. Serve the rolls as they are or toast in a panini press or hot oven for a few minutes.