Make meatballs: Preheat oven to 350 degrees F and line a large baking sheet with parchment paper. In a medium bowl, combine pork, cassava flour, 2 tablespoons oil, fennel, ¾ teaspoon salt, and ¼ teaspoon pepper. Using your hands, mix ingredients gently but thoroughly, until well combined. Using a small ice cream scoop or spoon, portion into 16 balls, then roll and place on baking sheet. Bake for 15 minutes.
While the meatballs bake, prepare the cabbage-apple sauté: In a large skillet over medium heat, melt 1 tablespoon butter and 1 tablespoon olive oil. Add onion, sprinkle with ¼ teaspoon salt and a pinch of pepper and cook, stirring occasionally, until tender, about 5 to 7 minutes. Raise heat to medium-high and add apple and cabbage. Season with 1 teaspoon salt and ½ teaspoon pepper and cook, stirring often, until all ingredients are very tender, about 6 to 8 minutes. Stir in vinegar and sauté for 2 minutes. Taste and season with additional salt and pepper, if desired. Transfer to a serving bowl and cover to keep warm.
Melt remaining 2 teaspoons butter and ½ tablespoon oil in hot skillet. Place meatballs in skillet and cook, shaking pan often to turn meatballs, until lightly browned, about 2 to 3 minutes. Serve meatballs with cabbage mixture. Sprinkle with parsley, if desired.