Season beef with salt and pepper, then sear to brown in oil along with the onion, either using a skillet or the sear function of the instant pot. With the amount of beef, I find it faster in the skillet.
Stir remaining ingredients (except cornstarch) and pour over the ribs in the pot.
Seal the lid and cook on manual/high for 45 minutes.
Use the quick release and do not remove lid before the steam subsides.
Remove ribs and skim any fat from the top of the sauce.
Mix a cornstarch slurry of 3 tablespoons starch with 1/2 cup cold water.
Stir into the sauce and set the cooker to sear, stirring sauce until it thickens up.
Serve the ribs over corn polenta, mashed potatoes, rice, etc. Polenta is shown in photos.