Preheat the oven to 350°F (175°C). Lightly grease a 9x9-inch baking dish or casserole pan.
Wash, peel, and chop 3 pounds carrots. Place the chopped carrots in a medium pot and cover with water. Simmer over medium heat for 15 to 20 minutes, or until fork-tender.
While the carrots are cooking, heat 1 tablespoon vegetable oil in a large skillet over medium-high heat. Add 1 medium white onion (diced) and cook for 7–10 minutes, until it becomes translucent and lightly browned. Season with 1 teaspoon salt.
Drain the cooked carrots and mash them to your desired texture, using a potato masher for a chunkier result or a food processor for a smoother consistency.
In a large mixing bowl, stir together 8 ounces sour cream, 1 cup shredded cheddar cheese, 1/2 cup grated parmesan cheese, and 1 large egg Mix well. Add the mashed carrots and cooked onion to the bowl and gently fold until evenly mixed. Transfer the mixture to the prepared baking dish.
Crush 1 sleeve Ritz crackers and place crumbs into a small bowl. Add 2 tablespoons butter (melted), 1/4 cup grated parmesan cheese, and 1 teaspoon garlic powder and stir to combine. Sprinkle the topping evenly over the carrot mixture.
Bake uncovered for 20–25 minutes, or until the topping is golden brown. Serve hot.