Preheat your oven to 375°F (190°C). Because a hot oven is waiting for no one, especially hungry humans.
Prep those peppers! Cut the tops off the bell peppers and remove the seeds and membranes. If they wobble too much, slice a tiny bit off the bottom to help them stand upright – just don't cut through to the inside. Nobody likes a leaky pepper.
Place the hollowed peppers in a baking dish. Pour the 1/4 cup water into the bottom of the dish (this creates steam to help soften the peppers). Set aside while you make the filling.
Heat a tablespoon of olive oil in a large skillet over medium heat. Add the onion and cook until softened, about 3-4 minutes. Add the garlic and cook for another 30 seconds until fragrant. Your kitchen should be smelling pretty fantastic right about now.
Add the ground beef to the skillet and cook until browned, breaking it up with a wooden spoon as it cooks, about 5-7 minutes. Drain excess fat if necessary. Season with salt and pepper.
If using zucchini, add it now and cook for another 2 minutes until slightly softened.
Stir in the tomato paste, drained diced tomatoes, Italian seasoning, smoked paprika, and red pepper flakes (if using). Cook for 2-3 minutes, allowing the flavors to blend together.
Remove from heat and stir in the cooked rice, half of the shredded cheese, and the chopped parsley. Taste and adjust seasoning if needed. The filling should be moist but not soggy.
Stuff each bell pepper generously with the filling mixture. Don't be shy – really pack it in there! Top each pepper with the remaining shredded cheese.
Cover the baking dish with foil and bake for 25 minutes. Then uncover and bake for another 10 minutes until the peppers are tender and the cheese is golden and bubbly.
Let stand for 5 minutes before serving. This rest time is non-negotiable unless you enjoy burning the roof of your mouth (I don't judge, but I do warn).