| 1 | Pre-heat oven to 300 °F, 150 degrees c, gas mark Bake the tortillas directly on the oven shelf for 15 to 20 minutes, turning once until crisp right through. |
| 2 | They will break with a clean snap. |
| 3 | Break into coarse pieces and set aside. |
| 4 | Spread onion pieces out in a heavy frying pan. |
| 5 | Cook over moderate heat without stirring until the onions are sizzling and beginning to stick to the pan. |
| 6 | Stir in the stock and let it bubble up, stirring up the browned deposits in the pan as it bubbles. |
| 7 | Stir in the garlic and the spices. |
| 8 | Turn the heat down and simmer, stirring frequently until the mixture is thick, not at all soupy, and the onions and spices are frying in their own juices. |
| 9 | Don"t rush this step. |
| 10 | It is essential that the spices should not have a harsh raw taste. |
| 11 | Stir in the black beans and heat gently. |
| 12 | Mash roughly while still in the pan, with a potato masher. |
| 13 | You want a rough lumpy mixture, not a smooth puree. |
| 14 | Set aside. |
| 15 | In a gratin dish spread a layer of a third of the tortilla pieces. |
| 16 | Spread a layer of the black beans over the tortillas and spread a third of the tomato sauce over the black beans. |
| 17 | Sprinkle with one third of the cheeses. |
| 18 | Repeat this, ending with a layer of the beans, sauce and cheeses. |
| 19 | Bake for 30 minutes covered. |
| 20 | Uncover, and bake for 5 to 10 minutes more. |
| 21 | Serve garnished with lime wedges |
