Brown the ribs: Pat the pork ribs dry and season with salt and pepper. Heat bacon grease or oil in a large Dutch oven over medium-high heat. Sear the ribs in batches for 3–4 minutes per side, until deeply browned. Transfer to a plate.
Cook the onions: Reduce heat to medium. Add the sliced onions and cook for 15–20 minutes, stirring frequently, until deeply golden. Add the butter and stir until melted.
Make the gravy: Sprinkle the flour over the onions and add 2 additional tablespoons of butter and cook for 2 minutes, stirring constantly. Slowly whisk in the beef broth, scraping up the browned bits from the bottom of the pot. Stir in Worcestershire sauce, apple cider vinegar, Dijon, smoked paprika, onion powder, garlic powder, thyme, and black pepper. Add Better Than Bouillon, Liquid Smoke, and red pepper flakes, if using.
Braise: Return the ribs and any accumulated juices to the pot. Cover, reduce heat to low, and cook for 2–2½ hours, or until the pork is fork-tender and nearly falling apart.
Thicken the gravy: If needed, remove the ribs. Whisk 2 tablespoons cornstarch with ¼ cup cold water. Slowly whisk the slurry into the simmering gravy and cook for 1–2 minutes, until thickened.
Serve: Taste and adjust seasoning. Serve the tender ribs over creamy mashed potatoes with plenty of onion gravy. Garnish with fresh parsley, if desired.