What to use instead of cheese?

An egg in the white sauce and some nutmeg

All brilliant ideas, thank you! I think I could sneak in a bit of parmesan as it's not runny and gloopy. I know, from discussing this with him numerous times, that it's the texture that is more of a problem than the actual taste. As I said before, he can eat "raw" cheese -even mature Cheddar but not blue cheese. That is a step too far! He even eats Camembert sometimes but "hot melted" cheese is a no-no! Onion sounds good, and herby breadcrums too. I will be experimenting with all of your suggestions over the coming weeks and will report back!

Mustard powder and Engebita yeast flakes added to white sauce has a nice "cheesy" taste

What about using vegan cheese? Does that upset your H's stomach?

Herby breadcrumbs scattered over the top add a bit of extra texture. If your husband can tolerate grated parmesan cheese, then you could mix a bit of that into the breadcrumbs too.

Onion sauce. Parsley sauce.

Mushroom sauce instead of white sauce. I make it for GS on pasta someones. It's yummy.

Thank you all! I'll try some of these to ring the changes. It's one of the reasons I've never eaten a take-away pizza, and Italian restaurants are an occasional treat although they are very good at leaving out cheese if requested. I dream of trying a Domino's pizza, believe it or not!!!

Already some good ideas. Stirring something into the white sauce is a good idea as it can lift the sauce and it doesn't have to be the same ingredient/s each time. Pesto or houmous are some of the additions I have added.

I'm vegan so I use Engevita nutritional yeast instead of cheese. It looks like flakes of fish food but has a savoury, cheesy taste. It's best sprinkled on after cooking as high heat can destroy it's vitamins.

Crushed chickpeas

Add plenty of seasoning and mustard powder to the white sauce then top with breadcrumbs and chopped parsley.

If you mix an egg into your white sauce first ? For the top bit I mean.