One thick slice of bread with one tablespoon of mustard yields a medium-thin sauce that is lightly flavored. Doubling those ingredients thickens the sauce and makes it spicier. With flour on the meat as it browns and two slices of bread, the sauce is quite thick. If you like those proportions but prefer more sauce, thin it out by adding more beer toward the end of the cooking time.
For slow cooking, add the cooked ingredients to the slow cooker on low for about 6 hours.
For oven cooking, put the covered casserole in a low setting. Simply Recipes, a site I trust, recommends 300 degrees F for carbonnade.