Preheat oven to 375 degrees F. Cut the squash in half. Cut a thin slice off the round side of each squash half to create a stable base. Sprinkle with salt and pepper; coat with cooking spray. Place squash flesh side down on a baking sheet lined with aluminum foil; bake until golden and tender, 40 minutes. Remove from oven; flip squash and set aside.

In a large nonstick skillet over medium heat, heat 1 teaspoon oil. Add sausage; cook, breaking into coarse pieces, until brown, 6 minutes; transfer to a bowl. To same skillet, add remaining 2 teaspoons oil and leek; cook until leek is soft, 3 minutes. Add garlic; cook, 30 seconds. Add kale and toss; add broth. Cover and cook until kale is tender, 5 minutes; stir in sausage. Divide kale-sausage filling among squash.

In a bowl, combine walnuts, Parmesan, panko and optional red pepper flakes; sprinkle evenly over squash bowls and coat with cooking spray. Broil until panko is golden, 2 minutes.