Preheat the oven to 350 degrees.
In an oven safe skillet (i.e. cast iron), add the olive oil. Place over medium heat on the stovetop. Allow the oil to become completely hot and shimmering.
Add the onions, sprinkle with a pinch of salt, and saute until they are translucent and light brown.
Add the squash, sprinkle with salt and garlic powder, and saute. Cook the squash down, allowing some of the water to cook out. This should take 10-15 minutes. The centers of the squash rounds should become translucent, and the rounds should be completely flexible but not falling apart.
Turn off the stovetop burner. Add the butter by dotting it all around the skillet, stirring it into the squash and onion mixture.
Top the mixture with cheese.
Mix the panko with a small amount of olive oil, about a teaspoon. Spread this mixture over the cheese.
Cook at 350 degrees for 25-30 minutes until the cheese is melted and bubbling and the breadcrumbs are browned. Serve immediately.