Position racks in the top and bottom thirds of the oven and heat the oven to 425°F. Line two rimmed baking sheets with parchment.
In a medium bowl, toss the sprouts with the olive oil and season generously with salt and pepper. Spread the sprouts out evenly on the two baking sheets. Roast until the sprouts are tender (the best way to test is by tasting), 20 to 22 minutes, stirring the sprouts once or twice during roasting. (Note that any loose leaves will be browning deeply; if they seem to be actually burning, turn down the heat a bit or just pick out the burned leaves.)
While the sprouts cook, combine the butter and shallot in a small, heavy saucepan. Cook, stirring often, over medium heat until the butter is melted and the shallots are soft, about 2 minutes. Raise the heat to medium high and continue cooking, swirling the pan, until the milk solids in the butter turn golden brown and the butter smells very nutty (take care that none of the shallots burn), about 2 minutes. Whisk in the lemon juice, thyme, lemon zest, mustard, and salt and pepper to taste. Keep warm until ready to dress the sprouts.
Slide the sprouts into a medium bowl, add the vinaigrette, and toss to coat. Season to taste with salt, pepper, or lemon, and serve immediately.
Make Ahead Tips
The vinaigrette can be made 1 to 2 hours ahead and kept warm.
nutrition information (per serving):
based on 6 servings, Calories
15, Fat Calories
130, Saturated Fat
4, Monounsaturated Fat
10, Polyunsaturated Fat
Photo: Scott Phillips