Boil water for the pasta. Add salt and turmeric. Peel onion and cut into small cubes. Put the bell peppers in a sieve and let them drain. Clean, wash and cut leek into small pieces.
Cook the cannelloni in the salt-turmeric water for about 4 minutes. Heat the oil in a pan and fry the minced meat in it until crumbly. Cut the peppers into small cubes. Remove the cannelloni from the water and rinse with cold water.
After 4 minutes add leek, paprika and tomato paste to the mince and fry for about 3 minutes. Add 100 ml of water and fry until the water has boiled away. Put the mince in a bowl. Drain the cannelloni and carefully fill with the minced meat.
Place filled cannelloni side by side on a tray and pour hot water over them until the bottom is covered. Keep the tray warm in a preheated oven (electric cooker: 100 °C/ convection oven: 75 °C/ gas: level 1).
Melt the fat. Stir in flour and sweat for about 5 minutes while stirring. Add stock and milk while stirring, bring to the boil and simmer for about 1 minute. Wash oregano and dab dry. Season béchamel with salt and pepper.
Arrange the cannelloni on the béchamel and garnish with oregano.