Peel and quarter the potatoes. Pluck the individual pieces from one of the three rosemary stalks. Mix together with the olive oil, salt, pepper and potatoes in a bowl. Place in the preheated oven at 200 ° C on baking paper and bake for 25 minutes. Prepare the trout using the 3-S method (clean, sour, salt). Dust with the flour on a plate. Melt the butter in a coated pan. Fill each trout with a rosemary stalk and fry for about 10 minutes on each side at medium temperature.