Put the turmeric, cumin, coriander, ginger, olive oil and 1 of the garlic cloves in a bowl and stir well. Add the tuna, coating all the pieces with the mix. Cover and leave in the refrigerator to marinate for at least 1 hour, preferably overnight.
Mix the yogurt with the remaining garlic clove and the lemon rind and season with salt and pepper. Heat a griddle pan over a high heat and brush with a little vegetable oil. Sear the tuna pieces in batches for 1 minute on 1 side and 30 seconds on the other. Remove from the pan and serve with bamboo skewers for dipping into the yogurt sauce.