To make meatballs: Line a baking sheet with foil.
Heat oil in a large skillet over medium heat. Add onion. Cook until translucent, 7 to 10 minutes. Stir in apples, ginger and garlic. Cook 1 to 2 minutes. Remove from heat. Cool.
In a large bowl, combine pork, egg, breadcrumbs, milk, maple syrup, fennel, red pepper flakes, salt and black pepper. Add onion mixture. Mix with your hands until uniform. Roll by tablespoonfuls into 1-inch balls. Arrange on prepared sheet. Refrigerate 30 minutes.
Preheat oven to 400 degrees.
To make glaze: Whisk together maple syrup, tomato paste, apple juice and vinegar in a small saucepan. Bring to a simmer. Cook over medium heat, stirring occasionally, until reduced by half, about 5 to 7 minutes. Remove from heat. Cool slightly.
Brush meatballs with half of glaze. Bake 10 minutes. Brush with remaining glaze. Bake 5 to 10 minutes longer or until cooked through. Serve warm.