Preheat your oven to 425°F.
On a baking sheet, coat the mushrooms in the oil and season with salt. Spread them in a single layer and roast until browned at the edges and tender, about 20-30 minutes.
While the mushrooms are roasting, bring the stock to a simmer in a medium saucepan over medium-high heat, then reduce the heat and keep it at a bare simmer.
Melt the butter in a medium heavy pot over medium heat until it froths. Stir in the onion and cook, stirring occasionally until translucent, about 4 minutes.
Add the farro and cook, stirring for 1 minute. Stir in the wine, and simmer until the wine has almost completely evaporated, about 3 minutes.
Add about 4 cups of your stock and cook, stirring frequently until the stock is almost entirely absorbed. Add more stock one cup at a time and stir frequently until the farro is tender (about 25 minutes for semi-pearled and about 45 minutes for whole-grain).
Stir in the mushrooms and let simmer for about 1 minute. If you'd like the farro risotto to be saucier, add in some more stock or hot water. Remove from heat, then stir in the Parmesan cheese and creme fraiche. Season with salt to taste and a splash of olive oil. Garnish with chives or scallions. Serve and enjoy immediately.