Preheat the oven to 350 degrees F.
Heat a large oven-safe skillet skillet or dutch oven over medium-low heat. Add the butter and once melted, stir in the onion and garlic with a pinch of salt and pepper. Cook for 5 minutes.
Stir in another tablespoon of butter and add the cauliflower. Cook, stirring often, until the florets are golden and toasty, about 5 to 6 minutes. Add the remaining tablespoon of butter. Stir in the rice and rosemary. Cook for 1 to 2 minutes, until the rice is translucent. Add in the stock, wine and another generous pinch of salt and pepper. Stir until combined.
Place the skillet in the oven and cook for 35 to 40 minutes, until the liquid is absorbed. After 40 minutes, if the rice is still too al dente, add a bit more liquid (maybe 1/2-1 cup or so) and cook for another 10ish minutes until the rice is soft.
Remove the skillet from the oven and immediately stir in the hot water, parmesan cheese and chopped brie cheese. Taste and season with more salt and pepper if necessary. Sprinkle some more fresh rosemary on top and serve immediately.