Preheat your oven to 375 degrees F and spray a 9x13 casserole dish with non-stick spray.
In a 12-inch high-sided skillet set over medium-high heat, add olive oil.
When the oil is hot, add the thinly sliced squash and onion. Saute gently until the squash is soft but not falling part. You want to cook out the liquid but you're not looking to brown the squash or the onion. This step will take about 10 minutes.
Add in the garlic and saute for 1 minute.
Spoon the squash mixture into a fine mesh colander set over a bowl or the sink. Gently push on the squash to help it drain any remaining water. This step helps prevent the casserole from becoming too watery while baking.
While the squash drains, work on the second step of your filling.
In a large bowl, whisk the eggs and then whisk in 1/2 cup shredded cheese, sour cream, kosher salt, black pepper, and Italian seasoning.
Add the drained squash mixture to the egg/sour cream mixture and stir gently to combine.
Add the squash mixture to the prepared pan.
In a small bowl, mix together the melted butter and crushed butter crackers until evenly coated.
Top the casserole with 1/2 of the cracker mixture, then sprinkle with remaining 1/2 cup of shredded cheddar cheese, and then the rest of hte cracker mixture.
Add to the preheated oven and bake for 35 minutes or until the top is golden brown and toasted.
Serve warm, store leftovers in a covered container, and eat within 3-4 days.