Hollowed-out squash earns kid appeal when filled with a mix of beef, bell pepper, and corn, all topped with Parmesan cheese.
Heat oven to 400 degrees, with rack set in the top third. Halve each squash lengthwise; slice a sliver from the rounded part of each half so it sits flat. Leaving a 1/4-inch border, scoop out halves with a small spoon; roughly chop flesh, and reserve. Place squash halves, skin side down, in a shallow baking dish. Season with salt and pepper, and set aside.
In a large skillet, heat oil over medium-high. Add reserved chopped squash, bell pepper, scallion whites, and garlic; season with salt and pepper. Cook, stirring occasionally, until bell pepper begins to soften, 3 to 4 minutes. Add tomato paste and chili powder; cook, stirring, until fragrant, 1 minute. Add beef; cook until no longer pink, breaking up meat with a spoon, 2 to 3 minutes. Remove from heat. Stir in corn and 1/4 cup Parmesan; season with salt and pepper.
Dividing evenly, spoon mixture into squash halves. Sprinkle with remaining 2 tablespoons Parmesan. Cover with aluminum foil, and bake until squash is tender, 30 to 35 minutes. Uncover, and bake until top is browned, 7 to 10 minutes more. Garnish with scallion greens.