Peel and finely chop the onion. Wash the chicken filet, dab dry and cut into very fine cubes. Wash and chop parsley. Heat oil. Fry onion, chicken filet and minced meat in it at high heat for about 4 minutes.
Season with salt, pepper and sherry, stir in parsley and breadcrumbs. Fill cannelloni with it. Fill any leftovers into the casserole dish later. Melt the fat and sweat the flour in it. Add milk while stirring constantly and bring to the boil.
Melt 50 g Gouda in it. Season with salt, pepper and nutmeg. Wash, clean, quarter and seed the tomatoes. Cut the flesh into small cubes. Cut ham into small cubes as well.
Grease the casserole dish and spread half of the sauce in it. Place cannelloni on top and sprinkle with diced tomato and ham. Spread the rest of the sauce and cheese on top. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 40 minutes.
Serve in portions garnished with fresh herbs and olives.