Keto replacement for flour
One option is foregoing thickener entirely. I regularly make sauces with those proportions and no thickener and find them perfectly fine, texture wise.
Another option is substituting the half and half with full cream or even switch 1/3-1/2 of it for cream cheese. Or, similarly, add more parmesan or another cheese to thicken the sauce. All of those substitutes will make for a thicker sauce without any further additions.
Otherwise, xantham gum is keto friendly. You want to add about 0.2% xantham gum by weight to slightly thicken a sauce, so about 1.5 grams for your recipe (I recommend weighing your sauce on a kitchen scale and calculating the amount precisely). I would mix it well with a small amount of cream like for a cornstarch slurry, before mixing that into the rest of the liquid. Like Stephie pointed out in the comments, mixing any powder into a hot liquid is very difficult.