Preheat the oven to 350°F (175°C). Lightly coat a baking sheet with olive oil cooking spray.
Add the ground beef, salt, black pepper, paprika, Italian seasoning, garlic powder, cumin, grated onion with its juice, and egg white to a large bowl.
Mix gently until the ingredients are evenly combined. Avoid overmixing, which can make the beef filling too firm.
Cut each mini pepper in half lengthwise. Remove the seeds and any large pieces of membrane.
Divide the beef mixture among the pepper halves, gently pressing the filling inside and keeping the tops relatively flat.
Arrange the Keto Cheeseburger Stuffed Peppers on the prepared baking sheet with the filling facing upward.
Bake for about 20 minutes, or until the peppers have softened and the ground beef is fully cooked to a safe internal temperature of 160°F (71°C).
Sprinkle the shredded mozzarella evenly over the stuffed peppers.
Return them to the oven for another 3–4 minutes, until the cheese is melted and bubbly.
Garnish with chopped parsley if desired and serve warm.