3 cups Creme Fraiche (recipe below)1 1/2 tablespoons prepared Dijon-style mustard3 tablespoons tomato paste3 tablespoons Worcestershire sauce2 teaspoons imported sweet paprika3/4 teaspoon saltfreshly ground black pepper, to taste1 teaspoon demiglace1 pound medium-size firm white mushrooms10 tablespoons (1 1/4 sticks) sweet butter24 medium-size white pearl onions3 pounds beef tips (filet)chopped Italian parsley (garnish)Creme Fraiche:1 cup heavy cream (not ultra-pasteurized)1 cup dairy sour creamWhisk heavy cream and sour cream together in a bowl. Cover loosely with plastic wrap and let stand in the kitchen or other reasonably warm spot overnight, or until thickened. In cold weather this may take as long as 24 hours.Cover and refrigerate for at least 4 hours, after which the creme fraiche will quite thick. The tart flavor will continue to develop as the creme fraiche sits in the refrigerator. (2 cups)
-Combine creme fraiche, mustard, tomato paste, Worcestershire sauce, paprika, salt, pepper to taste, and demiglace in a medium-size saucepan and simmer slowly for 20 minutes, or until sauce is slightly reduced. Remove from heat and let stand, covered, while completing recipe. -Trim stem ends of mushrooms and discard. Wipe mushrooms with a damp paper towel and slice thin. Melt 3 tablespoons of the butter in a medium-size skillet and saute mushrooms until tender and golden, about 10 minutes. Transfer to a bowl and reserve. -Cut a small X in the root end of each pearl onion. Bring a large kettle of water to a boil and drop in the onions. Blanch for 10 minutes, drain, and rinse under cold running water. Peel the skillet and saute the onions, stirring and shaking the skillet often, until they are lightly browned, about 10 minutes. Transfer onions to the bowl with the mushrooms. -Cut the meat into thin slices on the diagonal. Heat remaining butter in the skillet and saute the pieces of filet over high heat until just lightly browned, 3 or 4 minutes. Transfer pieces to a plate as each batch is browned. Recipe can be prepared to this point several hours before serving. -To complete, set sauce over medium heat and bring to a simmer. Add mushrooms, onions, and any accumulated juices from the bowl, and simmer for 5 minutes. -Add slices of filet and any accumulated juices and simmer just until meat is heated through, about 2 minutes. Serve immediately, garnished with chopped parsley.