Clean and wash broccoli, peppers and leek. Cut broccoli into florets, dice peppers, cut leek into diagonal slices. Cook broccoli for about 5 minutes in boiling salted water. Add diced peppers and leek slices after 2 minutes.
Rinse the vegetables in a sieve with cold water. Drain the corn, add it and mix everything carefully. Soak gelatine in cold water for about 5 minutes. In the meantime stir yoghurt with mustard until smooth.
Peel the garlic, press it through a garlic press or chop it very finely and add it. Season to taste with salt, pepper and chilli powder. Dissolve squeezed out gelatine in a water bath and stir into the yoghurt mass.
Fold the prepared vegetables into the yoghurt mixture and pour everything into a round mould (1.5 litres capacity) rinsed with cold water. Leave to set in the refrigerator for 4 hours. For the sauce, puree chunky tomatoes with the cutting stick of the hand mixer.
Season to taste with salt and pepper. Wash the chives, cut into rings and stir into the sauce, except for 2 tablespoons. Carefully remove the terrine from the edge of the mould with a knife. Dip briefly in hot water, turn out on a plate and sprinkle with the remaining chives.
Serve with the sauce.