Gather all ingredients.

Preheat the oven to 350 degrees F (175 degrees C). Split ribs down the center between the bones.

Mix onions, ketchup, water, vinegar, brown sugar, Worcestershire sauce, mustard powder, and salt together in a bowl. Set aside 1/2 of the sauce for basting.

Heat oil in a large skillet over medium-high heat. Working in several batches, add ribs and sear until browned, 4 to 5 minutes per batch.

Transfer ribs to two baking pans and arrange in a single layer. Pour remaining BBQ sauce over ribs.

Bake ribs in the preheated oven, turning and basting with sauce every 20 minutes and adding reserved sauce as needed, until meat easily pulls away from the bone, about 2 hours.