Season chicken on both sides with kosher salt and black pepper.
Heat a large non-stick skillet to a medium-high heat. Add butter. Once butter is melted, add sliced mushrooms, toss in butter, then leave alone until mushrooms are browned. Stir and brown on the other side. Add 1/4 teaspoon salt, stir. Remove from pan.
Turn the heat down to medium and add remaining butter to the same pan. Once the butter melts, add chicken. Cook chicken until brown on both sides, about 4-5 minutes per side. Remove chicken to a plate and cover with foil.
Add sherry to the pan and simmer until reduce by half. Add lemon, cream and spinach to the pan. Stir until the spinach starts to wilt, then add chicken and cooked mushrooms back into the pan. Simmer for 3-4 minutes until sauce has thickened, spinach has wilted and chicken is cooked all the way through. Season to taste with salt and pepper.