Heat oven to 400° F.
Heat the oil in a large skillet over medium-high heat. Add the onion and cook for 2 minutes. Add the eggplant and bell peppers and cook for 3 minutes. Add the garlic, tomatoes and their juices, salt, black pepper, red pepper, and 1/4 cup water and 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 30 minutes. Stir in the chopped parsley. Divide the sausages and ratatouille among individual plates and add the parsley sprigs.
Tip: This dish has flavor to spare, so you don't need all the sodium that comes in a standard can of tomatoes. Try using a brand with no added salt instead.