Preheat the oven to 375°F (190°C).
Bring a large pot of water to a boil. Carefully remove the core from the cabbage and place the whole head into the boiling water.
Cook for about 3–5 minutes, gently separating the outer leaves as they soften. Remove and set aside.
Heat olive oil in a skillet over medium heat.
Add diced onions and cook for 3–4 minutes until softened.
Stir in garlic and cook for another minute.
Add ground beef and cook until browned.
Stir in cauliflower rice, Italian seasoning, salt, nutmeg, and parsley. Cook for 4–5 minutes until the cauliflower softens and excess moisture cooks off.
Remove from heat and mix in the whisked egg.
Spread about ½ cup marinara sauce on the bottom of a baking dish.
Place a portion of filling onto a cabbage leaf and roll it tightly, folding in the sides.
Arrange the rolls seam-side down in the baking dish.
Pour the remaining marinara sauce over the top.
Cover with foil and bake for 35–40 minutes until the cabbage is tender.
Let rest for a few minutes before serving.