Put the flour in a bowl with butter and a pinch of salt. Knead quickly to obtain crumbs. Keep the mixture aside.
Clean the vegetables and dice them. Fry the chopped onion in a pan with a little oil. Add the vegetables and cook them on a high flame for 15 minutes. Season with salt, pepper and basil leaves. In the meantime, dice Fontina PDO, removing the rind.
Oil the cocotte and transfer the vegetables on its base. Add Fontina PDO cubes and cover with the crumble. Press the surface lightly and bake in a preheated oven at 200°C for 25 minutes.