Preheat the oven to 350°F (175°C).

In a large skillet, heat the olive oil over medium heat. Add the sliced squash and chopped onion. Sauté for about 5-7 minutes until the squash is tender and the onion is translucent.

Remove the skillet from heat and stir in the salt, black pepper, garlic powder, and half of the cheddar cheese. Let the mixture cool slightly.

In a separate bowl, whisk together the milk and beaten eggs. Add this mixture to the cooled squash mixture and stir until well combined.

In a greased 9x13 inch baking dish, layer half of the crushed crackers on the bottom. Pour the squash mixture over the crackers, spreading it evenly. Top with the remaining crushed crackers and the rest of the cheddar cheese. Sprinkle the grated Parmesan cheese on top.

Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and bubbly.

Remove from the oven and let it cool for about 5 minutes before serving. Garnish with fresh parsley if desired.