Preheat the oven: preheat the oven to 180°C / 350-375°F / Gas Mark 4-5.
Dry rub: add the mustard powder, garlic powder, onion powder, paprika, cayenne pepper, salt and pepper to a jug and use a mini whisk to combine. Then set aside.
Beef ribs: remove the ribs from the packet, rinse them and pat them dry with paper towel. Then working one rack at a time flip the ribs so that the membrane (silver skin) faces you. Fold a piece of paper towel into a square and use this to grip one corner of the membrane. Pull it up and off then discard. Repeat for the 2nd rack.
Rub into ribs: Working one rack at a time sprinkle the dry rub onto the ribs and use your fingers to massage in. Then flip and do the other side. Repeat for the 2nd rack.
Baking dish: line 2 baking trays with aluminium foil and lightly spray the foil with oil. Then place the dry rub coated rib rack into one oiled tray each.
Cooking liquids: pour half of the cooking liquids into the 1st tray and the other half into the 2nd tray. Cover each tray with a layer of tightly pulled aluminium foil.
Oven bake: place each tray into the preheated oven for an initial 1 hour period, once the hour is up, use oven mits to remove the tray and place it onto a heatproof surface. Pull the foil off and then top up the liquid in the base of the tray with water. Place the foil back on the tray and then return the trays to the oven. Repeat the top up with water each hour (You will do this 3 times, the ribs bake for 3 hours and 30 minutes).
Grill/broil: use oven mits to remove the cooked ribs from the oven and place on a heatproof surface. Change the oven setting to grill at 200°C 392°F6. Use kitchen tongs to transfer each rack to a silicon baking mat on an oven rack and tray. Use the pastry brush to coat each rack with barbecue sauce. Place the tray into the oven on the top shelf. Grill the ribs for 10 mins. Then remove, repeat the basting process followed by a further 10 mins grill time (Grill time total: 20 minutes).
Serve and enjoy: use oven mits to remove the baking tray from the oven and place on a heat proof surface. Let the ribs rest for 2 to 3 minutes before using tongs to transfer them to a chopping board and use a sharp knife to cut them into individual ribs. Place the cut ribs onto to a serving plate. Provide Horseradish Yogurt Sauce (optional) for serving.