Place each chicken breast between 2 sheets of plastic wrap. Using a heavy skillet or mallet, pound the breasts to a thickness of ¼-inch. Season both sides with salt and freshly ground black pepper and lightly coat with a dusting of flour.

Place 1 tablespoon each of olive oil and butter in a large skillet over medium high heat. When the butter begins to foam, add two of the chicken breasts and sauté on one side until golden brown, 2 to 3 minutes. Turn the chicken breasts over and sauté the other side until cooked through, 2 to 3 minutes. Transfer the chicken to a plate, raise the heat to medium high and repeat with another tablespoon each of olive oil and butter and the other 2 chicken breasts.

Add the shallot and pears to the pan and cook over medium-high heat until lightly translucent and golden, about 3 minutes. Add the lemon juice, mustard, chicken stock, and any juices on the plate and de-glaze the pan, scraping any brown bits on the bottom with a wooden spoon. Simmer until the sauce reduces by half, about 4 minutes. Add the chopped thyme and parsley, and gradually stir in the remaining tablespoon of butter until just melted.

Spoon sauce over chicken cutlets and serve over spinach.