Bring 2 quarts of salted water to a boil in a large saucepan. In a small bowl whisk the flour together with 6 tablespoons water. Pour the flour mixture into the saucepan, along with the lemon juice. Cut the salsify into matchsticks and boil until crisp-tender, about 10-15 minutes. Drain and refresh under cold running water . Dry the salsify with paper towels and place it in a large buttered gratin dish. Preheat the oven to 475F. In a small saucepan whisk the cornstarch with 1/2 cup of water. Stir in the butter and crÅme fraöche. Season with salt and pepper. Heat gently without boiling. Pour the cream sauce over the salsify and sprinkle with the cheese. Bake for 15-20 minutes, until bubbling and golden. Serve immediately.
