Remove the thigh bone and put chicken legs aside while you prepare the filling. In a medium saucepan over medium-high heat, heat the olive oil with the garlic and cook until the garlic starts turning amber coloured, about 2 minutes. Add the leeks, onion, carrots, fennel, and celery.
Preheat the oven to 400°F (200°C).
Reduce the heat to low and simmer, stirring occasionally, until the vegetables are soft, about 8 minutes. Season with salt and pepper to taste, toss with the herbs, breadcrumbs, and egg.
Lay the chicken thighs out on a counter with the skin side facing downwards. Put about a pinky sized about filling where the thigh bone used to be and roll it up. Use a toothpick or skewer to help keep the stuffing inside.
Butter a skillet and lay the chicken on top. Pour chicken stock around and roast for 45 minutes, or until the chicken is fully cooked. Let rest for 10 minutes before slicing.
To serve, slice the chicken thigh into 4 pieces and leave the leg intact. Arrange on warmed plates with a spoonful of zucchini gratin.