I grew up eating a lot of Italian food, sausage, peppers & onions included. I frequently make this combination for friends or to take to a pot luck.
I buy the Italian sausage links at a local Italian deli. I fry it up - no oil needed as they will create more than enough of their own grease. I leave it whole initially. I do make a small slice in the "skin" at each end so it doesn't look shriveled after it is cooked. After it browns a bit I either slice it lengthwise or slice it in cirlces (like a coin). I rarely leave it whole because I want to be sure the insides cook - guess it comes from always making sure pork is cooked thoroughly.
Meanwhile I cut up onions (I try to use sweet onions) & a variety of peepers - green, yellow, red & orange. Before I add the veggies, i drain the sausage. Then I cook it all together until the veggies are done like we like them. I either serve the meal as is or with rolls.