Heat olive oil in a 12-inch heavy-bottomed skillet over medium-high heat until shimmering.
Add the sausage rounds in a single layer and sear for 3 to 4 minutes per side until deeply browned and the fats have rendered.
Remove sausage from the skillet using a slotted spoon and set aside on a plate, leaving the rendered fat in the pan.
Increase heat to high and add the bell peppers, zucchini, and red onion to the skillet.
Sauté the vegetables for 6 to 8 minutes, stirring occasionally, until the edges are charred and the texture is tender-crisp.
Reduce heat to medium, add the minced garlic, Italian seasoning, salt, and pepper; cook for 60 seconds until fragrant.
Return the cooked sausage to the skillet and toss with the vegetables for 2 minutes to ensure even temperature distribution.
Remove from heat, garnish with parmesan cheese if desired, and serve immediately.