I make ours with diced bacon in the bottom of the dish, sometimes add diced onion too.
Cook the bacon and onions in the oven while the cauliflour is cooking - in boiling water to al dente. I use to cook the cauli whole but it's quicker broken into florets. Cheese sauce is roux based white sauce with lots of strong cheddar. put it all together, top with breadcrumbs and some grated cheese, put it back into hot oven until topping is crispy.
I used to do potatoes with it but even DP with a big appetite found it too filling!
I've never used mustard (DP once claimed he doesn't like it) but it sounds good. I might give it a go next time. I bet he won't even notice 
Breadcrumbs are quick and easy to make, whizz bread in food processor and dry out the crumbs in the microwave. I make them in quite large amounts and keep them in an airtight container. They won't spoil as long as they have no moisture left in them.
Retiring and living frugally in money from downsizing after years of stress



