2 1/4poundsbone in, skin-on chicken breastscut in half crosswise through the bone (side to side so you have two more “square” vs long pieces)—about 3 breasts
1 1/2tablespoonschopped fresh thyme
Place a rack in the center of the oven and preheat the oven to 400 degrees F. For easy cleanup, line a large, rimmed baking sheet with aluminum foil, parchment paper, or a silicone baking mat.
Place the onions, bread cubes, and garlic cloves in a large bowl. In a small heatproof bowl or liquid measuring cup, melt the butter. Whisk the olive oil, mustard, and 1 teaspoons kosher salt in with the butter, then pour over the onion-bread mixture. Toss to coat as evenly as possible. Turn the mixture out onto the prepared baking sheet. Spread into an even layer.
Nestle the chicken pieces into the onion mixture. It can overlap the onions a little, but make sure it does not cover the bread. Sprinkle the chicken with the remaining 1 teaspoon salt and pepper.
Bake for 15 minutes, then remove the pan from the oven and carefully turn the bread cubes. Return to the oven and bake until the bread is deeply golden and the chicken registers 160 degrees F on an instant-read thermometer inserted at the thickest part, about 15 additional minutes for smaller pieces and 20 to 25 minutes for larger pieces. If some chicken pieces cook more quickly than others, remove the cooked pieces to a plate and cover to keep them warm, then return the pan to the oven and continue baking until all of the chicken is cooked through.
If you'd like the bread to be even more crisp, you can transfer all the chicken pieces to a plate (cover the plate with foil to keep the chicken warm), then turn the bread once more and return the pan to the oven for 5 to 7 additional minutes, or until the bread reaches your desired crispness. Return the chicken to the pan and sprinkle the whole shebang with fresh thyme. Transfer the chicken to plates. Serve hot with the croutons and a spoonful of the roasted onions and garlic over the top.