Preheat the oven to 350°F Coat a baking sheet and an 8" x 8" baking dish with cooking spray.
Slice the acorn squash in half lengthwise, and use a spoon to scrape out the seeds from its center. Lay the 2 halves face down on the prepared baking sheet and bake for 35 minutes, or until the squash is very tender. Remove from the oven and let rest for 10 to 15 minutes.
Place a large nonstick skillet over medium-high heat and add the asparagus and 1/4 cup of water. Cover and let the asparagus steam for 5 minutes, or until the asparagus is bright green. Use a slotted spoon to transfer the asparagus to a small bowl. Drain the pan and wipe it clean.
Brown the beef over medium heat in the same skillet you used for the asparagus, breaking it up with the back of a wooden spoon as it cooks.
Remove the beef from the heat and drain any fat that has accumulated in the bottom of the pan. Add the corn and stir to combine. Season to taste with salt and pepper.
When the squash is cool enough to handle, scoop out the flesh with a spoon, and mash it with the back of a fork until it's smooth. Stir in the onion powder and season to taste with salt and pepper.
To assemble, spread half of the squash over the bottom of the casserole dish, followed by the ground beef mixture and then the asparagus. Top with the remaining squash.
Bake uncovered for 20 minutes, or until warmed through.