Toss the potatoes with the butter and mustard on a baking sheet until coated; season with salt and pepper. Roast, stirring once halfway through, until the potatoes are browned and tender, about 25 minutes. Stir in the spinach and garlic and continue to roast until the spinach is wilted, 1 to 2 minutes more. Adjust the seasoning with additional salt and pepper.
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