Adapted from A Cozy Book of Breakfasts & Brunches by Jim Brown and Karletta Moniz, Prima Publishing, 1997.
Remove the sausage from casings if you're using links and brown in a large pot, breaking up the meat into fairly small pieces. Remove sausage from pan with a slotted spatula, leaving fat behind. If there's not too much (judgment call here), then just leave it and use that as your fat for sauteing the onions and mushrooms. Pour off whatever you think is excessive. If, on the other hand, the pan seems dry, then use the optional butter or oil for the next step.
Saute the onions until soft, then add the mushrooms. Cook until almost all of the liquid has evaporated. This step is very important, since leaving the moisture in the mushrooms results in a soupy mess. Believe me, I know!
Meanwhile, grate the cheese and mix the eggs, milk and seasonings in a bowl. Spread the bread, sausage, vegetables and cheese in a 9x13 baking dish or a half-sheet chafing dish pan, mixing to distribute evenly. Pour the egg/milk mixture over this and make sure that all the bread is in contact with the liquid. I use a spatula and press down on the top. Cover the pan with foil. Then you can bake the casserole right away (let it sit at room temperature while the oven heats so that the bread has a chance to soak up all the custard) or refrigerate until you're ready to bake. Take the pan out of the fridge at least a half hour before baking if you do this.
Heat the over to 350 degrees and bake the casserole for about an hour, then take off the foil and continue baking until the top is browned and the middle is completely set. You don't have to remove the foil but I think it makes the top more attractive. Temp the middle to make sure that it has reached 1600.
*I usually use crimini mushrooms, small brown mushrooms that have more flavor than ordinary white mushrooms. (Although it's an interesting fact that crimini mushrooms are simply more mature white mushrooms. Their greater age makes them meatier and firmer, with fuller flavor. But ordinary white mushrooms will also do fine.) Rinsing, wiping, trimming and slicing fresh mushrooms takes some time, true. But the effort is worth the result!
**This recipe is basically what is usually called a strata, meaning layers, and the original version has you use slices of bread. But then you have to get the ingredients all spread out evenly between the bread. I think it's much easier to cube the bread and then just mix all the meat, cheese and vegetables up with those cubes, pack that mixture into the pan, and then pour the egg/milk mixture over. If you use a baguette you'll need two, or at least enough to equal about a pound. I've used King Soopers multi-grain French bread and Costco's country French bread with great success.
***You can add extra on the top, if you really like cheese. (And who doesn't?)
****The original recipe calls for Cheddar, and that's what I tend to use. If you have something else on hand, though, that's reasonably flavorful (Gruyere, for example), go ahead with that. Shoot for something medium to sharp. I wouldn't use a bland cheese such as mozzarella, but I also wouldn't use an extra-sharp Cheddar, as it will be overpowering.