Acceptance of health treatment claims that are not adequately supported by evidence. I dislike it when people sell ineffective and even harmful alternative health products to exploit the desperation of people with chronic illness.
I don't have a recipe but basically to avoid flour you'd have to replace it with some other thickening agent. It's hard to think of one that would have the right texture. The amount of flour in a serving of sauce would be tiny. If you have Coeliac disease, rice flour might work, I don't know.
I use cream cheese and cream in order to get the currect texture, without using a caby flour. But, if I'm looking for a way of doing something, I just google, in this case it might be "low carb mornay sauce" and see what comes up. There's usually loads, and I just pick what I fancy best. Having done that for your requirements, I landed on this one, which might well work for you?
I use creme freshe as I'm not a great lover of cream and I prefer the slightly acidic taste of creme freshe, warm the creme freshe add grated cheese and a dab of mustard stays nice a thick so it's a good coating sauce or one for making low carb lasagne etc. if you want a thinner pouring sauce you can let it down with a tot of cream
I bought the Dukan diet book in a charity shop and that has several no flour sauces using creme fraiche and this is my go to.
Cornflour is obviously a carb, but a little of it can thicken quite a lot of liquid which might work out les carby than using flour. Another less carby option might be to use a lower carb flour to make a roux or beurre manie.
The classic no flour sauces are Hollandaise and Bearnaise and they can be made with the aid of a blender.