Preheat your oven to 425° F. Peel and dice the red onions. Transfer them on a parchment lined baking sheet and drizzle generously with olive oil. Season with salt and pepper.
Bake the onions in the middle of the oven until golden brown and caramelized. Toss and turn them once or twice while they cook, about 30 minutes. Remove from the oven and set aside.
In the meantime, cook the Ravioli according to package instructions and drain. Season with salt and pepper. Add red onion rings, arugula and Parmigiano Reggiano. Toss to combine and serve.