Preheat the oven to 400F and lightly grease a 9x9 square glass baking dish.
Cook the squash and onion in a large pan with 1/3 cup water on medium-high heat for about 8-10 minutes or until soft. Strain any excess water before placing them in a large mixing bowl.TIP - You may want to allow the squash to sit in the strainer for a few minutes to get the majority of the water out, and lightly press them down without squishing them.
In a medium bowl, mix together the cheddar cheese, sour cream, eggs, salt, thyme, and ground black pepper. Pour into the bowl with the squash and combine well. If it becomes very liquidy you can drain some out.
Transfer to the prepared baking dish and cover with ¼ cup parmesan cheese.
In a small bowl, combine the crushed Ritz crackers and melted butter well. Sprinkle an even layer on top and bake for about 15 minutes or until bubbling and golden.
Choose squash that are firm and feel heavy with shiny skin.
You do not need to peel the squash for this recipe. The skin actually helps keep the squash rounds intact after being baked.
You can substitute the sharp cheddar with mild or extra sharp if desired. Easily swap out the cheddar entirely and go with your favorite kind of cheese, or a blend of the two.
The Ritz crackers can be substituted with breadcrumbs or saltines if preferred.
Squash has a high water content, which will shine through in the finished casserole if you don't drain it well enough before combining it with the other ingredients. You'll want to try and remove as much excess water as possible both during the sauteeing process and when you drain them afterward. Keep in mind the squash will still continue to cook in the oven after sauteeing so don't overcook them in the skillet, this can cause them to become mushy.