Cut vegetables into long, even strips. Melt butter in a pan, add lemon juice. Simmer the vegetables in this mixture for 2-2.5 minutes until they become soft but remain crispy. Salt, pepper and drain in a colander. Grate the tuna fillet with salt, pepper and chopped parsley, coat with olive oil and fry in a dry hot frying pan for 1 minute. from each side. Then quickly cut into cubes with a side of 1.5 cm. Put the cubes of tuna on a hot plate, place a tower of spaghetti on top of each cube.