Heat olive oil in a large skillet over medium heat. Add the sliced sausage and cook for about 5-7 minutes, until browned and cooked through.
Add the onion and garlic to the skillet, stirring frequently. Cook for another 3-4 minutes until the onion is translucent.
Toss in the sliced red bell pepper and broccoli florets. Cook for an additional 5-6 minutes, stirring often, until the broccoli is bright green and tender-crisp.
If desired, sprinkle in the red pepper flakes for an added kick and season with salt and pepper to taste.
Remove the skillet from heat and add the lemon zest and juice, mixing thoroughly to combine all flavors.
Top the dish with grated Parmesan cheese and let it sit for a minute for the cheese to melt slightly.