Add chicken thighs & baby potatoes to a metal roasting pan (or Pyrex / ceramic dish with a glass lid).
In a small bowl, whisk lemon juice, orange juice, and German mustard.
Pour sauce over everything.
Drizzle with Greek olive oil, season with salt, pepper, oregano. Add garlic if using.
Cover and bake at 400°F (200°C) for 1 hour.
Remove lid and bake uncovered for 30 minutes to caramelize.
