12 oz (375g) raw, peeled beetroot cut into 1/4"  (6.35mm) thick slices about 1/2" (12mm) wide (wear gloves, they stain!).
3 tablespoons vegetable oil, I sometimes use a little sesame oil as a substitute
1 large, fat, clove of garlic chopped as small as you can
1 heaped teaspoon of cumin seeds
1 teaspoon of corn flour to thicken, ordinary flour will do
Cayenne pepper - I use 1/4 teaspoon, this makes it subtly hot
1/2 lb (250g) chopped tomatoes, tinned will do but fresh are better
1 teaspoon salt
Some water, about 1/4 pint is usually enough but be prepared to add more if things get sticky.

Method:

Heat the oil in a heavy saucepan. When hot, add the cumin seeds and garlic. Stir and fry for a minute or so. Add the onion, continue stirring and frying. When the onions are just starting to brown, turn down the heat and add the cayenne, stir twice and add the corn flour or flour, keep stirring. The objective is to get the flour to absorb some of the oil. Do not get things too hot at this point. Add the tomatoes and salt. Stir and bring to the boil, add the beetroot. Bring back to the boil, cover and allow to simmer for about 40 minutes or until the beetroot is tender.
If the sauce is too thick, remove cover, take up the heat, and thicken whilst stirring.