Season the chicken with salt and pepper. Melt 1 tablespoon of the butter in a large skillet over medium-high heat, and when melted ad the chicken cutlets, in batches if necessary, cooking for about 3 minutes on each side, until cooked through. Remove the cooked chicken to a plate, and tent with foil.
Return the pan to medium-high heat and add the remaining tablespoon butter. Add the shallots and onions, and sauté for about 5 minutes until softened, and maybe slightly golden. Stir in the mustard then add the spinach (you will want to do this in batches, and let it cook down slightly in between additions) and cook, stirring often, for about 3 minutes until the spinach is just wilted. Season with salt and pepper, and stir in the lemon juice and zest.
Divide the chicken and spinach evenly between 4 plates. Add whatever other side dish you like, and serve hot.
Notes
Have your ingredients prepared before the chicken hits the pan. In fact, do yourself a big solid and prep them a day or two ahead of time. If you’ve zested and juiced the lemon, sliced up the shallots and onion, you are all set.