Prepare the Kamado BONO grill for indirect cooking, light the charcoal, place stainless steel grids, and wait until the temperature reaches 100 degrees Celsius. Place smoking chips on the coals. Adjust the top and bottom vents to maintain an even temperature. Place sliced rhubarb on the grids and smoke for about 20 minutes.
Place a frying pan on the grates. Add a splash of oil and sauté finely chopped onion and garlic. Add chopped lemon, sprinkle with sugar, and dissolve it while stirring. Add chopped rhubarb, tomato paste, and spices. Stir over very low heat until the rhubarb softens slightly. Optionally, you can blend the sauce with a blender or add crushed chili pepper if you prefer it spicier.
If there’s no BBQ/smoking option available but you really want this sauce, you can put the rhubarb in a frying pan and add a tablespoon of liquid smoke.