Heat The Peanut Oil In A Large Skillet Over High Heat. Brown The Ribs Evenly In The Hot Oil, About 5 Minutes; Remove From Heat. Stir The Tomato Sauce, Dried Onion, Hoisin Sauce, And Shiitake Mushrooms Together In A Slow Cooker. Add The Ribs To The Sauce Mixture, Turning To Coat In The Sauce. Cook On Low Until The Ribs Are Falling-Apart Tender, 8 To 10 Hours.