Preheat the oven to 400 degrees F (200 degrees C). Place potatoes in a single layer in a ceramic casserole dish.

Melt butter in a skillet over medium heat. Add garlic; cook until golden. Stir in rosemary; cook until just fragrant. Pour over potatoes; season with salt and black pepper.

Cook in the preheated oven, uncovered, until potatoes are fork-tender, about 30 minutes. Remove from oven; toss in spinach leaves. Return to oven; cook until spinach wilted, 1 to 2 minutes.