Cut off end of squash, slice in half, then again in half lengthwise. Cut into half circle slices.
Slice the onions into smaller sized pieces.
Add butter to a large sauce pan ( I use my 12” cast iron) and heat over medium heat.
Add onions and cook until soft, about 4-5 minutes.
Add squash, brown sugar, salt, and pepper. Stir, cover, and cook for about 20 minutes, until squash is very soft.
Uncover and continue to cook for 5 minutes to let the juices evaporate slightly. There will be some liquid in the pan but it should be thicker than water, almost like a glaze.
Season with more salt and pepper, if needed, and serve immediately.