Take the lobster meat out and dice up the meat.
Bring the stock, carrots, onions and tomatoes to the boil and simmer for an hour.
Melt butter add garlic, onion and crushed lemongrass and sweat, do not colour. Add barley and cook for 2 minutes. Add the stock a ladleful at a time until barley is almost cooked, approx 40 minutes. Add last stock, take out the lemongrass, add tomatoes, parsley and chopped lobster and season. Leave to one side.
Grill the tuna steaks with olive oil and season.
Put risotto in bowl with tuna on top, squeeze lemon juice over and serve.
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