Slow Cooker Sausage, Potato and Onion-Stuffed Peppers

In a 5- to 6-qt slow cooker, combine the crushed tomatoes, oregano, and 1/4 tsp each salt and pepper. If the peppers do not sit upright, slice a very thin piece off the bottom. Finely chop the piece and place in a large bowl. Add the onion, potato, parsley and crushed red pepper (if using) to the bowl and toss to combine. Add the sausage and mix to incorporate. Using a paring knife at a slight angle, cut the tops off the peppers; discard the seeds. Spoon the sausage mixture (about 1 cup each) into the peppers. Arrange the peppers upright in the slow cooker and place the tops over the filling. Cook, covered, until the sausage is cooked through and the peppers are tender, 5 to 6 hours on low or 3 to 4 hours on high. Using two large spoons, transfer the peppers to plates, letting any excess liquid drain into the sauce. Stir the sauce and serve with the peppers.