How to Make It
Preheat the oven to 425°. Heat a large, deep skillet until hot to the touch. Add the spinach and cook over high heat, tossing, until wilted, about 1 minute. Transfer the spinach to a colander and let cool slightly. Press out as much of the liquid as possible.
Season the chicken with salt and pepper and sprinkle with the dill. Place 2 slices of prosciutto on each chicken cutlet. Top with the spinach and place 1 piece of Boursin on each cutlet. Roll the chicken cutlets up lengthwise and secure in 2 or 3 places with toothpicks. Season with salt and pepper and dust with flour, tapping off any excess.
Wipe out the skillet, add the olive oil and heat until shimmering. Add the stuffed chicken and cook over high heat until golden on the bottom, about 4 minutes. Turn the cutlets and cook for 2 minutes longer. Add the wine and stock to the skillet, cover and bake in the oven for about 12 minutes, or until the chicken is cooked through. Transfer the chicken to a platter and remove the toothpicks. Cover with foil and keep warm.
Add the cornstarch mixture to the skillet and whisk over high heat until slightly thickened. Whisk in the remaining piece of Boursin and season with salt and pepper, pour the sauce over the chicken and serve.