Heat vegetable oil in a large skillet over high heat. Cook sliced squash in 2 batches, until it starts to soften. This will take about 5 minutes per batch. The ideal texture is slightly soft, but not falling apart.
Remove from heat and drain to remove any excess liquid.
Transfer to a large mixing bowl. Set aside.
In a small bowl, combine egg, whole milk, melted butter, sour cream, Kosher salt, ground pepper, garlic powder and ½ cup shredded cheddar cheese.
Gently toss cooked squash with this mixture.
Transfer to a 3-quart casserole dish prepared with cooking spray.
Top squash casserole with crushed crackers, remaining ½ cup shredded cheddar cheese and dried parsley.
Bake, uncovered, for 20-25 minutes until cheese has melted and top is bubbly and brown.
Remove and allow to cool for 5-10 minutes before serving.
Buttery crackers can be any type of buttery cracker, such as Ritz or Club crackers
Crackers don’t have to be perfectly crushed into crumbs, in fact, I like it best when there are varying sizes to make it a little more interesting.