Remove a portion of the breast using the modern procedure. Brown all over until pink inside. Trim, polish and season with salt.
Spring onion stock. Peel the spring onions and place on a baking sheet. Cover with aluminium foil and bake at 100º C for 6 hours so that they release any liquid. Strain through a cloth strainer and season with salt and sugar.
Choose ripe but firm fruits and cut into four pieces. Remove any seeds and separate the flesh from the rind. Cut the flesh into small dice and the rind into a fine julienne.
Place the portion of breast of tuna on a dish. Cover the base with the stock. Sprinkle the tuna with the fine slices of kumquat rind and add about 4 kumquat dice to the soup.