Turn on the broiler on the oven.
Bring water to a boil in a pot, add the cauliflower and cook until soft.
Remove as much of the portobello stems as possivle without damaging them. Be very gentle.
Put the portobello mushrooms on a aluminium foil covered baking sheet and bake the mushrooms for about 5 minutes on each side. Just until tender.
Meanwhile preheat a nonstick pan over medium heat.
Add onion and garlic to the pan and fry for 1-2 minutes.
Add the spinach and mangold and combine well.
Add the spices and roughly chopped walnuts and fry until the spinach is soft and tender.
Take out the portobello mushrooms from the oven and add the spinach filling. Don't press the filling down too much. Turn of the broiler function and bake on for another 2-3 minutes.
Meanwhile drain the cauliflower florets.
Mix using a hand mixer until smooth. I like to leave some chunks and not make it totally smooth.
Add dijon mustard and cumin and combine.
Place on plates, top with the portobello mushrooms and add some more roughly chopped walnuts on top (optional).