French Onion Chicken

very large yellow onion (thinly sliced (about 12 ounces or 3 1/2 cups))

good-quality sourdough bread (whole grain if possible, cut into 1 1/2-inch cubes (about 3 1/2 heaped cups))

garlic (peeled and left whole)

unsalted butter

extra-virgin olive oil

Dijon mustard

kosher salt (divided)

bone in, skin-on chicken breasts (cut in half crosswise through the bone (side to side so you have two more “square” vs long pieces)—about 3 breasts)

black pepper

chopped fresh thyme