In a wide saucepan, add a little olive oil and the spring onion, minced. Peel and cut the celery roots and potatoes, and add them to the saucepan. Pour the milk and add a sprig sage. Salt. Close the saucepan and cook over low heat. Meanwhile, prepare the tart base. Butter well the tart pan and lay the fresh KANAKI shortcrust pastry. Bake in a preheated oven at 160°C with extra heat on the oven elements, for 35 min. Bring the stuffing to a boil, remove the sage and add the grated graviera cheese and bacon bits. Beat the materials in the blender, add 3 egg yolks and stir. Empty the stuffing onto the tart base and sprinkle with grated cheese. Bake at a preheated oven at 170°C with extra heat on the oven elements, for 40 min.