Preheat the oven to 375°F. Butter a 2-quart baking dish.

In a medium bowl, combine half the crushed crackers with half the parmesan. In a large nonstick skillet, melt 2 tablespoons of the butter over medium heat. Pour the melted butter over the cracker mixture and stir to combine. Reserve for the topping. Do not wipe out the skillet.

In the same skillet, melt the remaining 4 tablespoons butter over medium heat. Add the onions, pepper, and 1 teaspoon of the salt, and cook, stirring frequently, until softened and translucent (do not let the onions start to brown), about 10 to 12 minutes. Transfer the onions to a large bowl and let cool for 5 minutes.

In a large measuring cup, whisk together the heavy cream, eggs, and remaining 1 teaspoon salt. Add the egg mixture to the onions, along with 1 ½ cups of the cheddar cheese, the remaining crushed crackers, and the remaining parmesan. Toss to combine. Spoon the mixture into the prepared baking dish and top with the remaining ½ cup cheddar cheese and reserved cracker mixture.

Bake until golden brown and bubbling, 30 to 35 minutes. Let the casserole sit for 5 minutes before serving.