Cut the leek in half lengthwise, then slice it into half-rings. Set aside.
Dice the bell peppers into small cubes. Set aside.
Clean and slice the mushrooms. Set aside.
Roll up all but one of the sage leaves and chop them finely.
Peel the celeriac and cut it into small cubes. Place it in a pot. Pour in the milk and water. Add salt and the remaining sage leaf. Cover and cook until completely tender.
Meanwhile, prepare the ragout. Dice the meat and sauté it in olive oil until golden brown. Season with salt and pepper.
Add the mushrooms and sauté together. Season again with salt and pepper.l.
Add the leek and bell peppers. Salt lightly and sauté until softened.
Once everything is nicely browned, sprinkle with the oat flour. Stir continuously to toast the flour slightly, then pour in the water. The sauce will begin to thicken. Stir in the cream as well.
Add the chopped sage leaves and simmer the ragout together. Once ready, pour it into an ovenproof dish and spread evenly.
Blend the cooked celeriac with an immersion blender until creamy. Add the sheep’s cheese, grated Parmesan, salt, and ground nutmeg. Mix well, then spread it evenly over the turkey ragout.
Bake in a preheated oven at 200°C (392°F) for about 40 minutes, or until the top is golden brown.