Preheat the oven to 425 degrees. Place racks in the upper and lower thirds of the oven.
Slice the bell peppers into strips about ¼-inch wide. Cut the onion into slices slightly wider than the peppers.
Drizzle the olive oil over the peppers and onions, sprinkle with oregano, crushed red pepper flakes, salt, and pepper and toss to coat. Divide the vegetables between two sheet pans.
Place the Italian sausage on the sheet pans with the peppers and onions and bake in oven. After about 20 minutes, flip the sausages and stir the vegetables around. Continue to cook for another 15-20 minutes, stirring occasionally, until the sausages are baked through, and the peppers and onions have caramelized.
Notes
Be sure to slice the onions wider than the peppers. The peppers will take longer to bake, so this will keep the onions from burning before the peppers are done.
You may need to rotate the sheet pans from top to bottom halfway through if one seems like it's baking faster than the other.
You can swap in spicy Italian sausage for the sweet.
If you don't have dried oregano, dried Italian seasoning is a good substitute.
Fresh basil or parsley are great optional toppings if you want to add something green.
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