Preheat the oven to 375°F. Line a rimmed baking sheet with parchment paper or a silicone baking mat.

Add the cauliflower, apples, shallots, rosemary, and 1 tablespoon of olive oil to a large bowl. Season with salt and black pepper and toss until evenly coated.

Spread the cauliflower mixture in a single layer on the prepared baking sheet. Roast for 25–30 minutes, or until the cauliflower is fork-tender.

While the cauliflower mixture roasts, heat the remaining tablespoon of olive oil in a large, high-sided skillet over medium heat. Add the garlic and cook for about 30 seconds, stirring frequently, until fragrant.

Add the Brussels sprouts to the skillet. Cook for about 15 minutes, stirring only occasionally, until golden brown and crisp around the edges. Add the cherries and cook for another 2 minutes, just until warmed through.

Transfer the roasted cauliflower and apple mixture to a large mixing bowl. Add the half & half and butter, then season with salt and black pepper to taste. Mash with a potato masher for a rustic texture or use an immersion blender for a smoother consistency.

Spoon the warm cauliflower-apple mash onto a serving platter or into a large bowl. Top with the crispy Brussels sprouts and warm cherries, then finish with the chopped pistachios. Serve warm.