Preheat the oven to 450 degrees F. Line a large baking sheet with aluminum foil and grease the foil with oil.
Arrange the zucchini halves on the prepared baking sheet. Add the cauliflower and spread it out in an even layer.
In a small bowl, stir together 5 tablespoons of melted butter, Cajun seasoning, and garlic until well mixed. Pour half of the butter mixture over the veggies, making sure to cover the cauliflower.
Bake for 15 to 20 minutes until the veggies are tender.
Place the sausage slices among the vegetables. Break up the crab legs and add them to the pan. Drizzle with the remaining butter mixture. Bake for an additional 10 minutes.
Squeeze the lemon half over the top, garnish with parsley, and serve immediately with remaining 5 tablespoons of butter for dipping.