How to Make It
Preheat the oven to 400°. In a food processor, pulse the bread until finely shredded. Add the garlic, rosemary and Parmesan, season with salt and pepper and pulse until combined. Add 2 tablespoons of the olive oil and pulse just until the crumbs are evenly moistened. Transfer to a small bowl.
In a medium, ovenproof skillet, heat the remaining 2 tablespoons of olive oil until shimmering. Season the chicken thighs with salt and pepper and add them to the skillet, skin side down. Cook over moderately high heat until golden, about 6 minutes. Turn the chicken and spread the skin with the mustard. Carefully spoon the bread crumbs onto the chicken, patting them on with the back of the spoon. Transfer the skillet to the oven and roast the chicken for about 15 minutes, until the crumbs are golden and crisp and the chicken is cooked through.
Meanwhile, in a medium saucepan, melt the butter. Add the onion and sugar, season with salt and pepper and cook over moderate heat until softened, 6 to 7 minutes. Add the lemon juice and cook until the liquid has evaporated, 2 minutes longer. Spoon the onion mixture onto 2 plates, top with the chicken and serve.