Small red or yellow potatoes can be substituted for the baby potatoes; cut them into bite-sized pieces before baking.
1 Heat the oven to 400°F. Place a rack in the middle position.
2 Prepare the chicken: With scissors or a sharp knife, trim the excess chicken skin and fat from the thighs. Sprinkle all over with salt and pepper.
3 Marinate the chicken in the sauce: In a large bowl, stir together the mustard, lemon juice, paprika, 2 tablespoons of the oil, and additional salt and pepper to taste. Add the chicken thighs and toss to coat. Set aside while you chop the vegetables.
4 Prepare the vegetables: Cut the baby potatoes into halves. Peel the red onion and cut it into 8 wedges through the root end. Cut the cherry tomatoes in half, but keep them separate from the other vegetables.