Preheat the oven to 350°F.
In a large oven proof baking dish big enough to hold the sausages snugly, lay them in one layer. Smear the sausages with an ample amount of Dijon mustard. Roast for 25 minutes. Remove from oven and turn the sausages over.
Toss the sliced fennel bulbs and the onion with a little olive oil, season with salt and pepper, then layer them on the sausages. Lay fresh thyme sprigs on top.
Return the dish to the oven and roast until the sausages are cooked through and the vegetables are tender, about 20-25 minutes. Remove from the oven, pour the red wine over, top with butter, and return to the oven for about 10 minutes.
Serve hot with white beans and greens, mashed potatoes, or with a simple green salad, which is how I usually prefer it. .
Bon Appétit!