The majority of French sausages, or saucisses, are made with 100-percent meat, give or take the spices, onion, and other flavorings, but certainly with no rusk or bread fillers. Increasingly, U.S. producers are starting to follow suit, given the demand for gluten-free sausages and also ones without any carbs, so it is now possible to buy a good selection in most supermarkets. — Anne Bell, Low Carb Revolution : Comfort Eating for Good Health.
Heat a tablespoon of oil in a large frying pan over medium heat and brown the sausages on both sides, in two batches if you need to, then transfer them to a large bowl. Arrange the onions, leeks, and thyme sprigs on the base of one or two roasting pans (I use a 10-by-15-inch pan, and a smaller 8-by-12-inch pan) — the vegetables need to be about an inch deep. Drizzle with the remaining oil and season, then arrange the sausages on top, spaced about 2 inches apart.
Preheat the oven to 375 degrees F and bake the sausages and vegetables for 1 hour, stirring every 20 minutes until the vegetables are silky and colored; they will reduce considerably in the process of cooking. Discard the thyme sprigs as you serve the sausages with the vegetables.
With Mustard Sauce: For a creamy mustard sauce to go with the sausages, while they are roasting, blend 1 cup sour cream with 1½ tablespoons Dijon mustard in a bowl (this can be done well in advance). You could also sprinkle in some coarsely chopped flat-leaf parsley.