Preheat the oven to 400°F (200°C).
Cut cauliflower into small florets and place in a pot.
Add water to cover the florets.
Add salt, about 1 teaspoon, and bring to a boil.
Parboil the cauliflower for a couple of minutes.
Drain the cauliflower and discard the water.
Chop the onion finely.
Place onion, mustard, mayonnaise and ⅔ parts of the cheese in a bowl and mix.
Add cauliflower and drained mussels to the mixture.
Blend together and pour into a greased baking dish.
Sprinkle the remaining grated cheese on top.
Bake for 15 minutes or until the cheese is bubbly and golden brown.
Serve with lettuce and olive oil.