Start by cutting up a whole chicken. Cut the breast pieces in half with a heavy, sharp knife.
Brush mustard on the skin side of each piece. Place the pieces in a plastic bag and refrigerate for at least one hour but up to 24 hours.
Clean and slice a head of fennel and remove the core and continue slicing the bulb. Slice zucchinis, carrots and celery. Place all the cut vegetables into a baking pan and toss with oregano, salt and pepper.
Remove the chicken from the plastic bag and place over the vegetables and season the meat with salt and pepper.
Bake in a preheated oven at 375 F for about 1 hour or until the internal temperature of the chicken reaches 170 F.