Thickeners

Arrowroot, letting the sauce reduce........ and a knob of butter makes any gravy thicker and glossier.

wiflib

clair - for stifado, use less liquid, the tomato puree and using less liquid plus finely chopped onions will help thicken the sauce, if its too runny after cooking, you can add a small amount of xanthan or guar gum, but I mean small amount or you will have a jelly instead of a glossy, creamy sauce. Or stir in cubes of ice cold butter, this will also thicken up a sauce.
You could at end of cooking time, remove the meat, puree sauce or part of it with your stick blender, then place meat back in the sauce.
One of these tricks will work, they do for me and I am no great big expert cook at all.
Hope you enjoy your stifado.