Brussels sprouts are finest in winter. Trim and cut the sprouts up to a day ahead, then keep refrigerated in a zip-top plastic bag.
Preheat oven to 400°.
Combine sprouts and chestnuts in a large bowl. Add oil, salt, and peppers; toss well to coat. Spread in a single layer on a jelly-roll pan.
Bake at 400° for 25 minutes or until sprouts are tender, stirring after 15 minutes. Remove from oven. Add butter to sprout mixture, tossing until butter melts.
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