Prepare the filling for the gratin: Heat a large, heavy-bottomed pot over low heat. Add the olive oil, apples, onions and curry. Season lightly with salt and pepper. Cook over low heat until the apples and onions become tender and translucent, 3 to 5 minutes.
Place a steamer basket over a pot of boiling water (or, if you have a steamer, simply use that) and steam the pieces of cauliflower until they are tender, 8 to 10 minutes. While the cauliflower is still warm, add it to the pot with the apple/onion mixture and stir to blend. Allow the vegetables to simmer gently on the stove until they are tender but still hold their shape, 15 to 20 minutes. If there is not sufficient liquid, add a little water to the pot so nothing burns as it cooks. Stir in the cream and allow the mixture to "rest" on the stove with the heat off, 5 to 10 minutes. Taste and adjust the seasoning, if needed.
Prepare the topping: In a medium skillet, melt the butter and stir in the garlic. Cook for 1 minute, stirring, and season with salt. Add the garlic powder and breadcrumbs and stir to blend.
Pour the cauliflower mixture into a gratin dish and top with half the breadcrumbs. Place in the oven and bake until hot and bubbly, 10 to 15 minutes. Remove and top with the Parmesan and the remaining breadcrumbs. Serve immediately.