In a medium skillet over medium heat, melt the butter. Add the shallot and cook until translucent, about 3 minutes. Add garlic and mushrooms. Cook until tender; remove to a bowl and set aside.
In same skillet, add broth and simmer until reduced by 1/3, scraping pan to release any mushroom bits.
Pour mushrooms back into broth; turn heat to low.
Combine flour, crème fraîche, tarragon, pepper, and a pinch of salt in a separate bowl. Add cream mixture to skillet, give it a good mix; cover the pot and cook another 5-7 minutes. (Note: if you're using sour cream, cook lower as it can curdle at high temperatures). Add salt & pepper to taste.
Meanwhile, bring a pot of salted water to a boil. Boil pasta until done, drain immediately. Divide amongst plates, topped with mushroom sauce. Garnish with italian parsley.