Place the chicken pieces in a big zip lock bag or some other container. Put the chicken in a bowl and pour the buttermilk and mustard over it. Cover tightly and let it sit for a couple of hours or overnight. Then coat all the surfaces.
Set up a metal-bladed food processor. Drop garlic cloves into the machine to mince. After turning off the machine, add crumbs, thyme, lemon zest, salt, and pepper to the bowl. Pour olive oil and pulse the machine to mix.
Skin-side down, press chicken pieces into crumbs in a pie pan or wide shallow basin. Marinade sticks crumbs to meat. Press crumbs on top by hand. Crumb-side up, place chicken on foil or parchment-lined baking pan. Use any remaining crumbs on bare chicken. Waste not!