Bring water, potatoes, cauliflower, garlic, and 2 tablespoons of the salt to a boil in a large saucepan over medium-high; reduce heat to medium and simmer, covered, until potatoes are fork-tender, 15 to 20 minutes.
While potatoes and cauliflower cook, place butter, thyme, rosemary, and sage in a medium skillet over medium; cook until butter is lightly browned, nutty, and infused with herbs, about 8 minutes, swirling skillet occasionally. Remove from heat and let cool slightly, about 20 minutes. Remove and discard herb sprigs. Set butter mixture aside, reserving 2 tablespoons butter mixture in a small bowl.
Drain potatoes and cauliflower; return to saucepan. Cook over medium, shaking saucepan occasionally, until no liquid remains, about 2 minutes.
Gradually add butter mixture in skillet to potatoes and cauliflower; using a potato masher, mash until combined. Add cream, pepper, nutmeg, and remaining ½ teaspoon salt, stirring until combined. Season with additional nutmeg to taste.
Spoon into a dish; using a spoon, swirl potato mixture to create crevices. Drizzle reserved 2 tablespoons butter mixture into crevices, and garnish with thyme leaves. Serve immediately.
