In a bowl, mix together dijon mustard, juice and zest from two lemons, chopped rosemary, pepper flakes, olive oil, and salt and pepper.
Whisk the mixture together. Place chicken in a baking dish. Brush the the mixture over the chicken, leaving a couple of tablespoons for re-painting during the roasting process.
Cover the chicken with the plastic wrap and refrigerate for at least 2 hours.
Preheat the oven to 400°F.
Slice 1 lemon and place it on top of the chicken, squeeze the juice from the remaining lemon all over the chicken and add few sprigs of rosemary on top.
Roast chicken for 35 to 40 minutes until the internal temperature reaches 165°F.