In a large frying pan, fry the chilli powder and flour in the oil for 1 minute, pour in the water, bring to the boil and cook, stirring, for about 5 minutes until slightly thickened; turn the heat down.
Dip a tortilla in the gravy for 20-30 seconds to soften, lay it on a plate, put 1 tbsp cheese and a little onion in the middle, roll up and lay seam side down in a baking dish.
If the gravy gets too thick towards the end, thin it with a little water; spread the remaining gravy, cheese and onion over the rolls.
Bake for 5-10 minutes until the cheese melts — no longer, or the cheese toughens — and serve hot.