To prepare the peppers:

1. Cut peppers into equal halves, cutting top to bottom, and then cut out the stem. Alternatively, you can leave the peppers whole and cut off the tops if you prefer. Remove the seeds and white membranes. Set the peppers aside while you prepare the filling.
2. Combine the tomato sauce and crushed tomatoes in a bowl. Whisk to combine and set aside.
3. Add the breadcrumbs, grated Parmesan, salt, pepper, oregano, basil, and fennel seeds to a separate bowl. Whisk to combine. Add the egg and 1/4 cup of the tomato mixture. Whisk again to combine thoroughly.
4. Add the ground beef, diced onion, and garlic cloves to the bowl containing the bread crumb and seasonings mixture. Mix everything until just combined. Hands work best here but you could also use a stand mixer. Don’t overmix or the meat may turn out tough.
5. Stuff the peppers with the meat mixture. Pack it tight in the peppers and don’t be afraid to heap up the meat a little bit.

If using the slow cooker:

1. Spread a few tablespoons of the tomato mixture into the bottom of the crock, then lay the stuffed peppers on a single layer on top. It’s OK if the edges overlap slightly, but you want to use a large enough slow cooker to comfortably fit the peppers.
2. Top the peppers with the remaining tomato sauce mixture, and then season them with a generous pinch of salt and pepper.
3. Cook on low for 6-8 hours, or on high for 3-4 hours.
4. About 30 minutes before you serve the peppers, remove the lid and spoon off any excess watery juices. Top with shredded mozzarella cheese, replace the lid and allow the cheese to melt for another 30 minutes.

To bake your peppers in the oven:

1. Set your oven to 325 degrees Fahrenheit. Spray a deep baking dish with cooking spray or coat with a thin layer of oil. Add a few tablespoons of the tomato sauce mixture to the bottom of the pan and place the peppers in a single layer in the pan.
2. Add the remaining tomato sauce to the tops of the peppers and season with a generous pinch of salt and pepper.
3. Bake for 1 1/2 -2 hours. Just before serving, remove the foil from the pan. If there are excess watery juices, spoon those off. Top with the shredded mozzarella cheese. Turn the heat up to 400 degrees Fahrenheit, and bake for another 15 minutes, or until the cheese is bubbly and golden brown to your liking.