Chop the onion and tomatoes and add to the flaked tuna fish. Crush the garlic and mix with the lemon juice, olive oil and mustard powder. Drain and thoroughly rinse the beans. Place in a saucepan and cover with fresh water and cook for 5 to 10 mins. Whilst the beans are still warm pour over the dressing. Add bean mix to tuna mix and season to taste. Serve with rocket.