Wash the sweet potatoes & prick them with a fork. Wrap in foil and bake for approx. 45 in a hot oven until potatoes are soft. Remove from the oven, allow cooling slightly, scoop out the flesh and discard the skin. 

Start cooking the pasta according to packet instructions, drain and set aside.

Prepare the sauce; melt the butter over moderate heat in a medium sized pot, stir in the flour to make a roux. Gradually, pour in the lactose free milk, stirring and cooking constantly till the sauce thickens. Next stir in the sweet potato flesh, garlic, mustard, cayenne pepper, nutmeg and salt to taste. Continue cooking for a couple of minutes until the sauce is thick. Whizz with a stick blender into a creamy smooth sauce, stir onto the pasta with the bay spinach and the finely sliced mushrooms, mix until veggies wilt. Add in enough lactose free cheese, pour into a baking serving dish, dredge with the remaining cheese, bake till cheese melts and golden.