How to Make It
Place half of the spinach in a large nonreactive ovenproof skillet. Cover and cook over moderately high heat, turning occasionally with tongs, until wilted, 2 to 3 minutes. Add the remaining spinach and cook, turning until wilted, 1 to 2 minutes more. Transfer the spinach to a colander and rinse with cold water until cool. Squeeze the spinach dry, then coarsely chop it.
Wipe out the skillet and return it to moderately high heat. Add 1 tablespoon of the butter and the garlic and cook, stirring, until fragrant and softened, about 3 minutes. Add the spinach, season with salt and pepper and stir for 3 minutes. Transfer the spinach to a bowl and let cool. Stir in the sun-dried tomatoes.
Place the chicken on a work surface, skin side down. Spoon the spinach mixture in a mound across each breast and press the Fontina cheese into the spinach. Roll the breasts around the stuffing and secure with 2 toothpicks; smooth the skin over the meat.
Preheat the oven to 400°. Season the chicken rolls with salt and pepper and dust with flour. Heat the olive oil in the skillet. Add the chicken rolls, seam side down, and cook over moderately high heat, turning, until browned all over, about 8 minutes. Transfer the skillet to the oven and roast the chicken rolls for about 15 minutes, or until the cheese is melted and the juices run clear when the meat is pierced with a knife. Place the chicken on a plate, discard the toothpicks and cover loosely to keep warm.
Pour the pan drippings into a small heatproof bowl and skim off the fat. Set the skillet over moderate heat. Add the stock, lemon juice and the degreased drippings and bring to a boil, scraping to dislodge any browned bits from the bottom of the pan. Boil until the liquid is reduced to 1/3 cup, 4 to 5 minutes. Remove from the heat and stir in the remaining 1 tablespoon butter.
Slice the chicken rolls crosswise 1/2 inch thick. Arrange on plates, spoon the sauce on top and serve.