Saute' the onions in olive oil over medium-low heat until caramelized, about 30 minutes.
Turn the heat up to medium-high and add the mushrooms. Saute' until the mushrooms are crisp around the edges, about 15 minutes. Season with salt and pepper.
Turn the heat to high and add the wine. Let the wine cook off for a couple of minutes, then lower the heat and simmer with the mushrooms for another 10 minutes. Stir in the butter 1 tablespoon at a time until combined. Remove from the heat and set aside.
In a large skillet, brown the sausage. When cooked through, about 5 to 7 minutes, add the mushroom mixture along with the tarragon and combine thoroughly.
