Rinse the basmati rice several times until the water runs clear, then soak it in salted water for at least 2 hours.
Bloom the saffron in hot water for 20 minutes.
Parboil the rice for 6–8 minutes, drain, and rinse with cool water.
Steam the rice with oil, butter, and saffron until fluffy with a crispy golden tahdig.
Combine the lamb, beef, onion, salt, pepper, turmeric, and baking soda. Knead for 10–15 minutes and refrigerate for 1 hour.
Shape the meat around flat metal skewers.
Grill over hot charcoal for 10–12 minutes, or broil in the oven for 12–15 minutes, turning frequently.
Brush the hot kababs with butter immediately after cooking.
Serve the kababs over saffron rice with tahdig, grilled tomatoes, fresh herbs, sumac, and warm flatbread.