Chop the mushrooms. (Optionally, reserve 3-4 large mushrooms to place in the center).
Heat 1 tbsp oil in a large frying pan. Add the onions and fry until lightly browned, about 3 minutes. Add the mushrooms, season with salt and pepper and fry until lightly golden, about 5 minutes. Then lift the mushrooms out onto kitchen paper to drain.
Place the pan back on the heat with the rest of the oil. Roast the garlic for a few seconds, add the spinach and cook for 2-3 minutes until wilted. Then place the spinach into a large sieve to drain and squeeze thoroughly. (Or simply thaw frozen spinach, and squeeze).
Combine fried mushrooms and onions with the cooked rice, season again to taste, add mustard (and optionally chopped walnuts and vegan cheese), stir to combine and place this mixture and the spinach in the refrigerator to cool completely.