Serve over noodles, rice, or potato puree.
Sprinkle veal with 2 teaspoons salt and 1 teaspoon pepper; sprinkle with flour, and toss to coat.
Heat 2 tablespoons oil in a large Dutch oven over high. Add veal, and cook, in batches if necessary, until browned on all sides, 5 to 6 minutes. Transfer veal to a paper towel-lined sheet pan. Add wine to Dutch oven, and stir, scraping up any browned bits from bottom of pot. Reduce heat to medium, and simmer until wine is reduced by half.
Cut and discard root ends and dark green tops from leeks, reserving white and pale-green parts only. Cut into 1-inch pieces, and rinse thoroughly.
Return veal to Dutch oven. Add leeks and next 7 ingredients, and cook, stirring often, until vegetables begin to soften, 8 to 10 minutes. Stir in 3 cups broth, and bring to a boil over high. Reduce heat to low, add saffron, and cover. Simmer until meat is tender, about 1 hour.
Meanwhile, bring remaining 1 cup broth to a boil in a saucepan. Add onions, 1 tablespoon butter, and 1/2 teaspoon salt. Reduce heat to low; cover and simmer until onions are tender, about 10 minutes. Drain over a small bowl, reserving onion liquid. Set cooked onions aside.
Melt remaining 2 tablespoons butter with remaining 2 tablespoons oil in a large skillet over high. Add mushrooms; sprinkle with remaining 1 teaspoon salt and 1/2 teaspoon pepper. Sauté until mushrooms are browned, about 5 minutes. Remove from heat.
Pour veal mixture in Dutch oven through a colander into a large bowl. Remove veal pieces, and reserve; discard vegetables. Transfer veal sauce in bowl to Dutch oven, and bring to a boil over high. Reduce heat to medium, and cook 5 minutes, stirring occasionally. Whisk in crème fraîche and Parmesan until blended and smooth, adding desired amount of reserved onion liquid if sauce is too thick. Stir in cooked veal, pearl onions, and mushrooms. Stir in chives and tarragon.