1. In a large saute pan over medium-high heat, melt 2 tablespoons butter. Add shallots, and saute until translucent, about 3 minutes. Add mushrooms, and cook until liquid is released yet mushrooms are still firm, about 5 minutes. Transfer shallots and mushrooms to a medium bowl; set aside. Remove pan from heat. Add 1/4 cup cognac to deglaze, stirring with a wooden spoon to scrape any browned bits from bottom of pan. Add cream, and return to heat. Cook until reduced slightly, about 5 minutes. Stir in mustard. Season to taste with salt and pepper.

2. Heat 4 tablespoons butter in the same pan over high heat. Working in batches, sear the meat for 1 minute, and add to mushrooms. Once all the meat is seared, deglaze saute pan with remaining 1/4 cup cognac, and return meat, mushrooms, and reduced-cream mixture to saute pan. Cook until just heated through, about 5 minutes. Add creme fraiche.

3. Heat remaining 1 tablespoon butter in a medium saute pan over medium-high heat. Add tomatoes, and saute until heated through, about 2 minutes.

4. Toss pasta with olive oil, saffron, tarragon, and parsley. Transfer to a large platter. Spoon beef Stroganoff over pasta. Garnish with tomatoes and tarragon sprigs.