Thinly slice about half the mushrooms and thickly slice the rest. Chop the veggie sausages into 3cm chunks. Heat 1 tablespoon of the oil in the casserole or frying pan. Add the thinly sliced mushrooms, veggie sausages and a pinch of salt. Cook for 20 minutes, stirring every few minutes. Mix in the pepper, thyme, paprika and Henderson's or Worcestershire sauce, if using. Turn the heat to low and cook for 5 minutes more, until the mushrooms are reduced and quite dry. Transfer to a plate and set aside.

Rinse the rice under cold running water and tip it into the saucepan. Put the pan on the hob, add the boiling water and a pinch of salt, cover and bring to the boil over a high heat. Stir once to loosen, then reduce the heat to a very low simmer. Cover and cook for exactly 12 minutes. Take the pan off the heat and set it to one side, leaving the lid on but with a small gap to let out steam.

Measure the mustard, milk and oats into the blender. Halve the lemon and squeeze in the juice. Blend together.

Trim, halve and slice the onion. Add the rest of the oil to the pan that you cooked the mushrooms in and turn the heat up to medium. Add the onions and cook for about 8 minutes, until softened and just starting to colour. Add the mushrooms, mix well, cover and leave to cook for 5 minutes more, mixing now and again. Reduce the heat to low, add the sauce and the rest of the mushrooms and heat through for 10 minutes.

Season the stroganoff to taste. Spoon the rice into serving bowls and spoon over the stroganoff. Sprinkle with fresh parsley and serve.