Cheesy Squash Casserole

1 tablespoon avocado oil or pork lard
4 medium thinly sliced yellow squash
3 medium thinly sliced zucchini
1 large thinly sliced yellow onion
4 tablespoons butter
1/2 cup grated Parmesan cheese
1 cup shredded sharp cheddar cheese
1/2 cup sour cream
1 sleeve crushed medium to fine buttery crackers
4 tablespoons butter (yes, again), this time melted

Preheat the oven to 350 degrees.
Heat the oil in a large skillet over medium heat. Sauté the squash, zucchini, onion and butter until soft. I chose an iron skillet. Stir the Parmesan, cheddar and sour cream in a bowl and add to sauteed veggies. Add salt and pepper to taste.
Place in the prepared casserole dish and sprinkle the cracker crumbs with melted butter evenly over the top. Bake for 20 minutes or until the top is golden and bubbly.