Preheat your oven to 375°F and line a rimmed baking sheet with parchment paper. Set aside.
Add the cauliflower, apple, shallots, rosemary leaves, and one tablespoon of olive oil to a large bowl. Season with salt, to taste, and toss to combine.
Spread the cauliflower mixture in a single layer on the prepared baking sheet and roast in the pre-heated oven until the cauliflower is fork tender, approximately 25-30 minutes.
Meanwhile, in a large, high-sided skillet over medium heat, heat a tablespoon of olive oil. Add the garlic and cook, stirring frequently, until fragrant, approximately 30 seconds.
Add the Brussels sprouts and cook, stirring occasionally, until golden brown and crispy, approximately 15-20 minutes. Add the fresh cherries and cook, stirring frequently, until warmed through, about 2 minutes. Remove from heat and set aside.
Once the cauliflower is done, transfer the roasted veggies from the baking sheet to a large mixing bowl. Add milk (or milk alternative) and remaining olive oil. Season with salt, to taste, and mash with a potato masher or blend with an immersion blender until mostly smooth. Set aside.
Spread the warm cauliflower and apple mash at the bottom of a large serving dish or bowl. Top with the crispy Brussels sprouts, warm cherries, and chopped pistachios before serving. Enjoy!