Preheat the oven to 400 degrees F. Line a rimmed baking sheet with aluminum foil.
Place the cauliflower, leeks, and red pepper on the prepared baking sheet.
In a small bowl, combine the olive oil, lemon juice, Dijon mustard, thyme, and salt. Add to cauliflower mixture, tossing gently to coat.
Bake until the cauliflower is tender, 35 to 45 minutes, stirring occasionally. Serve.