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Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Wash and dry produce.
Halve, peel, and dice onion into ½-inch pieces. Cut broccoli florets into bite-size pieces if necessary. Trim, peel, and cut carrots on a diagonal into ¼-inch-thick pieces. Zest and quarter lemon.
Toss onion, broccoli, and carrots on one side of a baking sheet with a large drizzle of oil. (For 4 servings, spread veggies out across entire sheet.) Season with salt and pepper. Roast on top rack until tender, 20-22 minutes (you’ll add more to the sheet after 12 minutes).