In a large skillet over low heat, melt 2 tablespoons of butter.
Sauté the onions for 30 minutes, stirring occasionally.
Add the sausage to the skillet, and cook until browned, 5 to 7 minutes.
Turn the heat up to medium-high, and add the mushrooms.
Season with salt and pepper.
Stir well and cook for about 10 minutes, or until the mushrooms begin to brown.
Add the wine or chicken broth to the skillet, and deglaze the pan, stirring to scrape up the browned bits from the bottom.
Lower the heat to low, and add the remaining 2 tablespoons of butter.
Stir well so everything gets incorporated.
Add the oregano, tarragon, and red pepper flakes, and stir again before serving.