Preheat the oven to 400. Line a half sheet pan with parchment. Use two pans if you don't have a half sheet pan.
Slice the sausages in to 1/2" thick pieces and add to the pan.
Core the bell peppers and slice in to 1/2" thick slices.
Peel the onion and slice in to 1/2" thick slices so they cook at the same rate as the peppers. Add the peppers and onions to the pan.
Drizzle over the avocado oil, salt and pepper and use your hands to toss everything together.
Roast until the sausages are sizzling and golden on the edges and the bell peppers are softened--about 20 minutes. Stir everything once to make sure it browns evenly.
Serve warm as it is, or pile on toasted buttered buns, add cheese and broil until melted and bubbly.
Notes
Nutrition calculation does not include rolls or bread.