Set oven temperature to 375 degrees.
Take the leeks' root off. Cut into thin rounds until the leek's dark green portion is almost reached. Place the rounds in a big dish, making sure to press them apart as you do so. Pour cold water into the basin and soak the leeks for ten minutes.
In the meantime, cut the little potatoes into uniformly sized chunks or quarters, approximately 1 inch. Slice the onion and reserve.
To prepare the vinaigrette, fill a small bowl with pyrek with a pour spout and add olive oil, lemon juice, 1 3/4 teaspoons kosher salt, 2 tablespoons thyme, and powdered mustard. Mix vigorously.
One teaspoon kosher salt, one-fourth teaspoon black pepper, and one teaspoon thyme are used to season the chicken breasts on both sides. Transfer to a baking dish.
Remove the leeks. Put the potatoes, onions, and peeled garlic cloves back into the bowl. Blend.
After giving the vinaigrette one more whisk, pour it into the bowl, then thoroughly mix to coat the potatoes and leeks. In case the salt has sank to the bottom of the plate, make sure you pour it all over. Add to the dish around the chicken.
Bake the chicken for 45 minutes to 1 hour, or until the internal temperature reaches 165 degrees.
Have fun!