Preheat your oven to 350°F. Lightly grease a large casserole dish with a bit of butter. In a large pan over medium heat, add 1/4 cup melted butter and the chopped onion; cook until soft, about 5 minutes.
Then add the chopped cabbage, and sauté for 8-10 minutes until the cabbage is wilted but still has a bit of crunch. Season with salt and pepper to taste.
Transfer the sautéed cabbage and onions into the prepared casserole dish. In a small bowl, mix the cream of mushroom soup with the Greek yogurt until smooth, then pour the mixture evenly over the cabbage.
In another bowl, toss together the shredded cheddar cheese and crushed whole wheat crackers with the remaining melted butter. Sprinkle this topping evenly over the casserole.
Bake in the preheated oven for 40-50 minutes until the topping is golden and the casserole is bubbly. Allow to cool for about 5 minutes before serving. If desired, garnish with fresh parsley.