Bring a large pot of water to a boil. Submerge the cored head of cabbage and blanch for about 3 minutes. Carefully remove the leaves with tongs, setting aside 8 large leaves for the burritos.

Thin the thick part of the cabbage leaves' core with a knife for easier rolling, if necessary.

In a large skillet, heat olive oil over medium heat. Sauté chopped onion until softened, about 6 minutes.

Stir in the taco seasoning and cook until fragrant, about 2 minutes.

Add the ground turkey to the skillet, breaking it apart as it cooks until it's no longer pink, about 4 minutes.

Pour in the tomato sauce and let the mixture simmer until the sauce thickens and the turkey is fully cooked, about 4-6 minutes. Adjust seasoning to taste.

Place two overlapping cabbage leaves on a flat surface. Spoon the turkey mixture onto the leaves, followed by sour cream, salsa, cheddar cheese, and avocado slices.

Fold the edges of the cabbage leaves over the filling and roll tightly into a burrito shape. Slice in half and serve warm.