• Preheat the oven to 350 degrees Fahrenheit and slice the onions and yellow squash.
• Then crush the Ritz crackers in a mixing bowl and set them aside for the topping later.
• Heat the butter in a cast-iron skillet over medium heat and, working in batches, cook the sliced squash and onions until they’re softened and lightly browned.
• As each batch finishes, set it aside in a mixing bowl and continue until all the squash and onions are cooked.
• In a separate mixing bowl, whisk together the eggs, sour cream, salt, pepper, cayenne, garlic powder, parmesan, and shredded cheddar cheese.
• Stir the cooked squash and onion mixture into the seasoned egg mixture until everything is evenly combined.
• Spray the inside of a 9x13 baking dish with cooking spray, then pour in the squash casserole mixture and spread it out evenly.

Make the Crunchy Cracker Topping

• Melt 3 tablespoons of butter and pour it over the crushed Ritz crackers.
• Add the remaining Parmesan cheese and shredded pepper jack, and stir until everything is well combined.
• Sprinkle the cracker and cheese mixture evenly across the top of the casserole.
• Bake the squash casserole at 350 degrees for about 35 minutes, or until the center is set and the edges are bubbling.
• Then switch the oven to broil on low, or about 550 degrees, to brown the top a little more. Watch it carefully here, because the cracker topping can burn quickly under the broiler.
• Once it’s done, remove it from the oven and let it rest for about 10 to 15 minutes before serving.