Peel off the outer leaves of the cauliflower and separate it into flowerets, rinse. Bring a large saucepan of salted water to a boil. When it boils, add the flour, mixed with 2 tablespoons water into the boiling water. Add the cauliflower and bring to a boil. Lower the heat so the water simmers and cook uncovered for 30 minutes. Peel the potatoes and cut into quarters. Boil for 15 minutes in salted water. Drain them and push through the fine mesh of a food mill. Drain the cauliflower and refresh under cold water. Preheat the oven to 425F. SautÄ the flowerets in 6 tablespoons butter. PurÄe in the food processor. Mix the 2 purÄes together. Add the eggs little at a time.Pour into a mold that fits into an oven pan. Dot with butter. Fill the oven pan halfway with hot water and cook for 1 hour. Prepare the sauce: place the egg yolks, vinegar, salt and pepper in a bowl fitted into a saucepan with 1-inch simmering water in it. Whisk thoroughly and add cubes of butter, little by little. Whisk continuously until you have a mayonnaise like consistency. Add teaspoons of lemon juice, taste for seasoning and rectify if necessary. Serve the sauce from a sauceboat with the mousse.
