Arrange two oven racks in the upper and lower thirds of the oven, then preheat to 450°F. Wash and dry the fresh produce. Cut off and discard the ends of the squash. Halve lengthwise; using a spoon, scoop out and discard the pulp and seeds. Cut crosswise into 1/4-inch pieces. Halve and peel the onion; cut into 1/2-inch-wide wedges. Thinly slice the scallions, separating the white bottoms and hollow green tops. In a bowl, combine the ketchup and as much of the gochujang as you'd like, depending on how spicy you'd like the dish to be. Season with salt and pepper.  

Meanwhile, line a separate sheet pan with foil. Evenly coat the center of the foil with a drizzle of olive oil. In a bowl, combine the beef, sliced white bottoms of the scallions, breadcrumbs, and egg. Season with salt and pepper. Gently mix to combine. Transfer to the oiled portion of the sheet pan. Shape into a tightly packed loaf, about 7 inches by 3 inches. Transfer half the glaze to a separate bowl and set aside for serving. Evenly top the meatloaf with the remaining glaze. Place on the upper oven rack and bake 16 to 18 minutes, or until the meatloaf is browned and cooked through.* Remove from the oven and let rest at least 2 minutes. 

*The USDA recommends a minimum safe cooking temperature of 160°F for beef.

Carefully transfer the rested meatloaf to a cutting board; slice crosswise. Serve the sliced meatloaf with the finished vegetables. Top the meatloaf with the reserved glaze and sesame seeds. Enjoy!

Place the squash pieces and onion wedges on a sheet pan. Drizzle with the sesame oil; season with salt and pepper. Toss to coat and arrange in an even layer. Place on the lower oven rack and roast 21 to 23 minutes, or until lightly browned and tender when pierced with a fork. Transfer to a bowl. Add the sliced green tops of the scallions and ponzu sauce; toss to combine.