Put the butter in a large skillet over medium heat. When melted, add the rosemary and garlic and let sizzle gently until fragrant, about 2 minutes. Don’t let the rosemary brown; adjust the heat as needed.
Add the celeriac; it should be in a single layer on the bottom of the skillet with some space between pieces (work in batches if necessary). Cook, turning the celeriac a few times, until it’s soft and golden brown on all (or most) sides, about 30 minutes. (Remove the rosemary and garlic when it gets browned.)
Sprinkle with salt and pepper and the minced rosemary; stir and serve.