Heat half of the oil in a large frying pan over medium heat. Add the onion, season with salt and cook for about 5 minutes, stirring occasionally, until softened. Throw in the shredded sprouts and stock, then let it bubble over a medium-high heat for a few minutes, until the liquid has reduced by about half. Add the sprout leaves, toss it all to combine, season and keep warm.
Cook the tortelloni according to the instructions on the pack and drain, reserving a ladelful of the pasta cooking water. Add the pasta the to pan with the onions and toss gently, then stir through half of the cheese and drizzle over rest of the oil, adding some of the reserved pasta water to loosen the sauce, if needed.