You want about a pound and a half of sausage total. These days, if you buy your sausage from a butcher or quality meat counter, they are much longer than the classic 5 inch link that you still see in packages in grocery stores. American super-sizing.... Anyways, I get my sausages at Whole Foods, and for this dish I get the Hot Italian variety, but using the Sweet Italian would be great too. I wait until they are on sale at $2.99/lb and then I load up the freezer. Truly, you can make this dish with any type of sausage. I once used some wonderful rabbit sausages from the Burdick Grocery in Walpole, NH. So yummy, but so out of my budget these days. If you are going to use a non-pork sausage, be sure it has a high fat content (of added pork fat or otherwise). If the sausages are not fatty enough, they will dry out in the oven.
Other than the sausage you'll need 8 cloves of garlic, 2 onions, about 8 ounces of white button mushrooms, 2 red peppers and 4-5 cubanelle peppers (also known as Italian frying peppers). Use the flat side of your knife and the heel of your hand atop it to whack the garlic to crush it a little and get the skin off. I've been using this knife method all my life, since watching my mother do it. However, I've recently discovered equally successful results by pounding my olive oil cruet atop the cloves. Onions get sliced in one-inch thick slices, as do the peppers. Mushrooms get quartered (or halved if really small).
This is sort of an assembly-line cooking situation. If you are in a rush, you can prep each ingredient while the previous one is cooking, but I prefer to prep everything, and then leisurely cook everything while sipping some wine or a refreshing orange juice with seltzer. It feels civilized.
So, take out your big skillet (regular, not nonstick). Pour in a couple TBS of olive oil and heat over medium high heat. Prick the sausages all over with a fork and brown in pan for about 10 minutes, turning a few times to brown them all over. Half way through cooking the sausages, add the garlic, turning frequently until golden brown. Transfer sausages and garlic to a large, deep baking dish. Leave oil in pan.
Add onions to pan and cook for about 7-8 minutes, stirring frequently, until lightly browned, but still a bit crisp. Transfer onions to baking dish with sausages and garlic. Add mushrooms to pan, and cook for about 4-5 minutes until browned and transfer to baking dish. Add a little extra olive oil to the pan (the mushrooms will have soaked up most of what was left after the onions), and add the peppers. Cook for about 7-8 minutes stirring frequently, until slightly wilted, but still a bit crisp and add to baking dish.
Put baking dish, uncovered, in a 400 degree oven for 30 minutes, tossing vegetables and turning sausages every ten minutes. Eat straight up with a good crusty bread on the side to dip in the olive oil and vegetable juicy goodness or make grinders with a nice roll. Obviously, this dish is easily shrinkable or expandable. But unless you don't like having an easy dinner in the freezer, it is usually worth making the whole thing, even for one person. If you prefer yellow peppers to red or don't like mushrooms, this is also very customizable. If you are thinking of substituting mushroom varieties, I have found that white button mushrooms are sturdier than criminis/baby bellas and better maintain a separate flavor from the other ingredients. The criminis and such work fine if that's what you have on hand, but they are too delicate to bring the same earthy punch in this dish.
This dish also holds pretty well. You can make a double batch if you have company coming over and serve at room temperature (summer room temp, not winter room temp) or reheat gently in a 200 degree oven. If you have frozen a portion or two, put on a plate and defrost and then reheat in the microwave. Enjoy!