In a medium bowl, whisk together the onion, garlic, ketchup, brown sugar, Worcestershire sauce, mustard, salt, and 1/2 cup water. Whisk until the sauce is well combined, then set it aside.
If your ribs aren't already separated, cut them into individual pieces with one bone per piece. Season all sides of the English-cut ribs generously with salt and pepper.
Heat oil in a large skillet over medium-high heat. Working in batches if necessary, add the ribs meat-side down, making sure they aren't touching. Sear until a brown crust forms on all sides, about 1 to 2 minutes per side.
Transfer the browned ribs to your slow cooker. For the most even cooking, arrange them in a single layer to the extent possible.
Spoon the prepared sauce evenly over the top of the ribs, ensuring they are completely covered. Place the cover on the cooker and cook on Low for 6 to 8 hours. You’ll know they are ready when the meat is tender and pulling away from the bone.
Important Note for Instant Pot Users: If you are using the slow cooker function on an Instant Pot, do not use the "Low" setting. It doesn’t get hot enough to cook the meat safely or tenderize the ribs. Always use the "Normal" or "Medium" setting, which is the equivalent of "Low" on a traditional slow cooker. This ensures the beef reaches a safe temperature and becomes perfectly fall-off-the-bone tender.
Depending on the fattiness of your beef, you may need to spoon off some grease before serving. When serving, plate the ribs and spoon the warm sauce from the slow cooker over the top, passing any remaining sauce on the side.