How to Make It
In a small bowl, whisk the mustard with the vinegar. Gradually whisk in the 1/4 cup of oil until emulsified. Add the thyme and lemon zest and season with salt and pepper.
In a large skillet, heat the remaining 1 tablespoon of oil, until shimmering. Add the brussels sprouts and cook over moderately high heat, stirring occasionally, until crisp-tender and charred in spots, about 5 minutes. Remove from the heat and stir in half of the dressing. Transfer to a bowl. Serve warm or at room temperature, passing the extra dressing at the table.