Heat half of the oil in a pan, add the onion and fry until it starts to soften, then add the garlic, celery and carrot and cook until soft. Meanwhile, heat the remaining oil in a separate frying pan and fry the mince in batches, scooping each batch out with a slotted spoon and leave any excess oil behind.
Add the mince to the veg, then stir in the tomato purée and cook for 1 min. Stir in the passata and bring to a simmer. Cover and cook over a very low heat for 1½-2 hrs, then add the milk and cook for 30 mins. If you're making the base ahead of time, you can leave it to cool at this stage then freeze for up to a month. (Defrost fully before using in the next step.)
Put the mince base in a pan and add 1 tsp chipotle paste, bring to a simmer and taste it – add the other tsp if you need to. Stir in the black beans and cook for 5 mins, then stir in the coriander.
Warm the tortilla bowls in a low oven. Serve the bowls filled with chilli and add the avocado, a squeeze of lime, soured cream and cheese.