Combine the tandoori mix with the garlic, lemon juice, salt and yoghurt.
Spread the marinade over both sides of the fish and leave to marinate in the fridge overnight or for a minimum of two hours.
To make the sauce grate the cucumber and press it through a fine sieve to squeeze out any excess juice. Combine with 3/4 cup of yoghurt and 2 tablespoons of chopped fresh mint.
Heat a flat BBQ plate and sear the fish for 2 - 3 minutes on each side.
Transfer to the oven and cook for a further 5 - 7 minutes to cook through completely and prevent the fish from drying out.
Serve with steamed English spinach and brown rice.