Cut the tops off the peppers. Remove and discard the stems, then finely chop the tops; set aside. Scoop out the seeds and as much of the membrane as you can. Place the peppers cut-side up in a baking dish just large enough to hold them upright.
Heat 2 tablespoons of the olive oil in a large skillet over medium-high heat. Add the beef, season with salt and pepper and cook, breaking up the lumps, until the meat is cooked through and just beginning to brown, 8 to 10 minutes. Remove to a paper towel-lined plate to get rid of the fat.
Wipe out the skillet and add the remaining 2 tablespoons olive oil. Add the onions and chopped peppers and cook until beginning to soften, 3 to 4 minutes. Add the garlic and zucchini and cook for another minute. Add the tomatoes and season with salt and a pinch or 2 of red pepper flakes. Cook until everything is heated through, then stir in the beef and rice. Taste and adjust the seasoning. Stir in 1 cup of the cheese.
Fill the peppers with the rice mixture and top each with a sprinkle of the remaining 1/2 cup cheese. Pour a small amount of water into the bottom of the baking dish and drizzle the peppers with a little olive oil. Cover with foil and bake for 30 minutes. Uncover and bake until the peppers are soft and the cheese is melted and lightly browned, another 15 to 20 minutes.
Add a little bit of oil to a preheated pan. Add in the onions and garlic. Simmer over medium heat for a minute or two until they are fragrant. Add in the ground beef. Stir and simmer it for 4 or 5 minutes until it’s lightly brown. Season it with salt and pepper, stir it one more time, and turn off the heat.
Put the ground beef into a mixing bowl. Add in the rice, Worcestershire sauce, ketchup, and egg. Mix until everything is well combined.
Cut the tops of the bell peppers. Use a sharp knife to remove the core and seeds. Rinse them out with cold water. You might have to trim a bit off the bottoms to make them sit straight.
Place the filling in the bell pepper and fill them to the top. Put them in a casserole dish.
Put the stuffed peppers into a 350 degrees preheated oven for 25 minutes. Remove the peppers, grate some cheese over the top, and place them back in for an additional 10 minutes or until the cheese is melted and brown.
Place the stuffed peppers on a plate, serve, and enjoy.