Heat the oil in a large wok or saucepan and add the onion. Cook for a couple of mins to soften, then stir in the curry powder and cook for 1 min. Gradually add the coconut milk, stirring all the time, until saucy. Add 600ml water to the pan, then stir in the honey and soy sauce. Bring to the boil and cook for 5 mins.
Add the stir-fry vegetables, tofu and udon to the pan, cover and cook for another 3-4 mins. Taste, season and add a little more soy sauce, if you like. Divide between bowls and scatter with the spring onions.