In a large bowl toss together the cauliflower, water, 2 tablespoons of the olive oil, and salt. Spread the cauliflower on a parchment-lined baking sheet and roast for 20-25 minutes, until lightly browned.
Meanwhile, whisk together the hot sauce, melted butter, and lemon juice. Remove the cauliflower from the oven and toss with the hot sauce mixture. Roast for another 6-8 minutes.
In the bowl of a food processor, combine the goat cheese, mascarpone, remaining olive oil, and milk. Pulse until combined. Season to taste with salt and black pepper.
Spread both sides of each of the rolls with the goat cheese-mascarpone spread. Add a layer of lettuce and tomato slices to the bottom half of each roll, then top with the roasted cauliflower. Serve warm.