Trim the top and root end off each onion and peel. Bring a large pot of water to a boil and parboil the onions 8–10 minutes, until the layers begin to soften.

Cool slightly, then carefully hollow out the center of each onion, leaving 2–3 outer layers intact as the shell. Finely chop enough of the removed onion to make 1/2 cup for the filling.

Combine ground beef, cauliflower rice, egg, garlic, chopped onion, salt, and pepper for the filling.

Stuff the mixture into the hollowed onions.

Whisk crushed tomatoes, lemon juice, and sugar substitute together in a large pot.

Arrange the stuffed onions in the sauce, cover, and simmer 45–50 minutes, until the onions are tender and the filling is cooked through.