In a 12- by 17-inch baking pan, mix onions with 1 tablespoon olive oil, the garlic, and 1/4 teaspoon salt. Bake in a 375° regular or convection oven, stirring halfway through baking time, until onions are soft and browned, 25 to 35 minutes.
Meanwhile, in a large bowl, mix vinegar, apple juice, and remaining 1/4 teaspoon salt. Add onions, spinach, apples, goat cheese, and pumpkin seeds. Heat remaining 4 tablespoons oil in a small frying pan over high heat. When hot, pour over salad and mix to coat and wilt evenly. Season with more salt and pepper to taste. Serve immediately.