Peel oranges and grapefruits so that the white skin is completely removed. Cut out the fillets with a knife between the parting skins, collecting the juice. Mix orange and grapefruit fillets with grenadine liqueur.
Beat egg whites until stiff. Add sugar while stirring. Roast the almonds in a pan without fat until golden brown. Take them out and let them cool down. Spread the fillets and juice on four ovenproof baking trays.
Add the beaten egg whites. Bake in a preheated oven (electric range: 225 ° C/ gas: level 4) or under the grill until golden brown. Sprinkle with browned almond flakes.