Pan-Seared Sirloin and Savory Demi-Glace Sauce with Brussels Sprouts Amandine and Mashed Sweet Potatoes

Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Dice sweet potatoes into ½-inch cubes. Zest lemon until you have 2 tsp; quarter lemon. Thinly slice chives. Trim and halve Brussels sprouts.