Preheat the oven to 180C/160C Fan/Gas 4.
Bring a large pan of water to the boil with the bay leaves, fennel seeds, thyme stalks and a good pinch of salt – you need enough water to cover and cook the cauliflower and for the volume of water to total at least 400ml/14fl oz.
Cook the cauliflower in the water until soft and cooked through. Drain and reserve the cauliflower water to make the sauce. The fennel seeds are fine left in; remove the thyme stalks and bay leaves. Put the cooked florets in a large baking dish and set aside.
Return the pan to a moderate heat. Melt the butter, add the flour, combine and stir for a couple of minutes to make a roux.
Add 400ml/14fl oz of the hot herby cauliflower water gradually to the roux and whisk vigorously to ensure no lumps form. Turn down the heat and cook the sauce for about 10 minutes stirring often until the sauce has thickened to the consistency of double cream. Add the cream, vinegar, and mustard. Season, to taste, with salt and pepper.
Meanwhile, place a large frying pan over a moderate heat and add the oil. When hot, add the sausage meat and fry for 3 minutes until roughly broken up and browning in places. Add the garlic and continue to cook for 2 minutes. Add the bread and thyme leaves and give the pan a good stir to combine over a low heat for another 2 minutes. Take off the heat and add the cooked cauliflower pieces and radicchio (if using) and stir again.
Spread the contents of the pan in a large baking dish in one even layer. Sprinkle the parmesan over the top and pour the sauce over. Cook in the oven for 40-45 minutes until bubbling with some pieces of the bread on the top going crisp and golden brown.
Place a spoonful of the pudding on a serving plate and serve with salad.