Place the cauliflower in a rectangular baking dish (about 9x13 or 8x11).
Mix the mustard and mayonnaise together in a bowl and spread on top of the cauliflower. Use your hands to mix it together well.
Sprinkle cheese on top and drizzle about 1 tbsp. of olive oil over the cheese.
Bake, covered, for 50-60 minutes until cheese is melted and lightly browned, and cauliflower can be pierced with a fork. You can remove the cover during the last 10-15 minutes to help the cheese brown.
If the cheese isn't browned enough, pop it under the broiler for a minute or two to crisp it up;
I don't recommend using grainy Dijon mustard as the texture is not great in the cauliflower bake;
Remove the cover during the last 10-15 minutes of baking to brown the cheese.