Stir together the mustard, oil, vinegar, honey, garlic, 3/4 teaspoon salt and 3/4 teaspoon pepper in a large bowl. Toss the chicken with the marinade to coat, then transfer the chicken to a 9- by 13-inch baking dish. Roast, uncovered, until the chicken is browned and cooked through, about 45 minutes.
Transfer the chicken to serving plates. Add the spinach and butter to the hot baking dish, tossing with tongs to wilt the spinach and coat it with the pan drippings, about 2 minutes. Serve the spinach alongside the chicken.
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