Came across this low carb sauce in an old recipe book:

3 egg yolks
1/4 cup white wine vinegar
1/4 cup water
1/4 tsp salt
1 tbsp melted butter

In a bain-marie, add everything except the butter and cook, beating constantly for 5-10 minutes until fairly thick.

Remove from heat, stir in butter and serve immediately.

I had it with steamed salmon and it was nice for a change. If I have it again, I'll dial down the vinegar and try ACV instead.