Preheat the oven to 180 °C. Line an oven tray with baking paper.
Start beef
Peel and finely dice 1 red onion. Peel and mince 4 garlic cloves. Heat 1 tbsp olive oil in a large frypan over medium heat. Add half the onion. Cook, stirring, for 1 minute or until soft. Add the garlic, 1 tbsp Mexican seasoning and 2 tsp ground cumin and stir for 2 minutes. Increase heat to high and add 300 g beef mince and cook, breaking up lumps with a spoon, for 4 minutes or until browned.
Finish beef
Then add 2 tbsp tomato paste. Cook, stirring, for 1 minute. Stir through 185mlor¾ cupof water. Add 400 g brown lentils and bring to a simmer. Cook, stirring occasionally, for 8 minutes or until thickened.
Cook tacos
Place 8 taco shells in the oven and cook according to the packet directions.
Prep toppings
Meanwhile, adding to a medium bowl as you go - finely chop 2 tomatoes and 1 yellow capsicum. Deseed and finely slice 1 green chilli, if using. Add 1 tbsp lime juice and the remaining onion. Season with salt and pepper. Toss until combined. Finely slice ½ iceberg lettuce. Grate 50 g cheese.
To serve
Top the taco shells with the beef and lettuce, spoon over the salsa and sprinkle over the cheese. Divide between plates or serve on the table for everyone to help themselves.