Making the marinade -- Put the mustard, lemon juice and oil in a small bowl or glass measure. Whisk until thoroughly combined. Then whisk in the salt, pepper and Herbes de Provence.
Put the chicken thighs in a zip-lock bag. Pour the marinade over the chicken. Seal the bag and massage it until all of the chicken is coated. Refrigerate for at least 30 minutes, or for up to 24 hours.
Put the broccoli and/or sliced yellow squash on a baking sheet, and lightly coat them with olive oil plus salt and pepper. Refrigerate until you are ready to bake.
When you are ready to bake, preheat the oven to 425°F (220°C). Arrange the marinated chicken pieces among the vegetables on the baking sheet. Bake until the chicken is done and the vegetables are tender -- usually 25 minutes. Enjoy!