Prepare the Base

In a 5- to 8-quart slow cooker, place the sliced bell peppers and onion. Sprinkle with garlic powder, dried oregano, and crushed red pepper if desired. Add the Parmesan rind, if using. Season generously with salt and black pepper, then mix thoroughly to combine.

Sear the Sausages

Heat the olive oil in a large skillet over medium heat. Add the sausages, cooking and turning them occasionally until browned evenly on all sides, approximately 8 to 10 minutes. Once browned, transfer the sausages to the slow cooker.

Deglaze and Combine

Turn off the heat under the skillet and pour in the crushed tomatoes. Scrape the bottom of the skillet with a wooden spoon to loosen any browned bits. Pour the tomato mixture into the slow cooker over the vegetables and sausages. Gently stir the contents to distribute the sauce evenly.

Cook and Serve

Cover the slow cooker and set it to low. Allow the dish to cook for 4 to 6 hours, or until the vegetables are tender and the sausages are thoroughly cooked. Adjust seasoning with additional salt and black pepper if needed. Garnish with torn basil leaves before serving, if desired.