Preheat the oven to 425°F (220°C).
Cut the smoked sausage into slices about 1/4 inch thick.
Rinse the baby potatoes and halve them; cut any very large potatoes into quarters so pieces are similar in size.
In a large bowl combine the sliced sausage, halved potatoes, drained green beans, chopped onion, minced garlic, olive oil, Italian seasoning, salt, black pepper, and red pepper flakes. Toss everything together until the vegetables and sausage are evenly coated.
Spray a 9x13-inch baking dish with nonstick spray and transfer the mixture into it, spreading into an even layer.
Bake on the center rack for 40–45 minutes, stirring once halfway through, until the potatoes are fork-tender and edges begin to brown.
Remove the dish from the oven, taste and adjust seasoning with additional salt and pepper if desired.
Sprinkle with chopped fresh parsley and serve warm.