In a large, deep frying pan, heat up the oil to a medium heat and then fry the chopped onion for 3 to 4 minutes. Then, add the chopped red bell pepper to the pan, along with the powdered garlic, ground cumin, paprika, ground coriander, oregano, and chili powder, and fry for a minute or two until fragrant.
Add the minced beef to the pan, and fry until browned all over.
Add the canned tomatoes, the tomato paste, and the beef stock to the pan. Stir everything in and then leave it to simmer for 15 minutes uncovered, to boil off some of the liquid.
Turn off the heat and then stir in the sweetcorn and the black beans.
Preheat the oven to 400 F.
In a large ceramic oven dish, layer up half of the corn tortilla strips, then half the can of refried beans, and then half of beef mince mixture. Repeat this so that all of the produce is used up, and then top with the grated cheddar cheese. Bake in the oven for 30 minutes, until the cheese is bubbling and golden on top.
Serve hot, sprinkled with chopped fresh coriander, with a dollop of sour cream and some sliced avocado.