Prepare the herbed breadcrumb mixture by combining garlic, lemon zest, salt, pepper, and thyme in a food processor until finely chopped. Add day-old bread and pulse until resembling coarse crumbs.

In a large bowl, whisk together buttermilk and Dijon mustard. Submerge the chicken and coat well. Cover and refrigerate for at least 30 minutes.

Preheat the oven to 400°F (200°C). Remove chicken from the marinade, allowing excess to drip off. Place on a parchment-lined sheet pan and coat with the breadcrumb mixture.

Roast the chicken for about 40 minutes until breadcrumbs are golden brown and the internal temperature reaches 165°F (74°C).

Let the chicken rest for 5 to 10 minutes before serving to allow juices to redistribute.