Heat oven to 400° F. Heat the oil in a large ovenproof skillet over medium-high heat. Add the onion and cook for 2 minutes.
Add the eggplant and bell pepper and cook for 3 minutes.
Add the garlic, the tomatoes and their juices, 1 ½ teaspoons salt, ¼ teaspoon pepper, the red pepper flakes, and ¼ cup water. Bring to a boil.
Nestle the sausages in the skillet, spooning the vegetables over them.
Transfer to oven and cook, uncovered, until the sausages are cooked through, about 25 minutes. Stir in the chopped parsley.
Divide the sausage and ratatouille among individual plates and add the parsley sprigs.