Dry the chicken well and season both sides generously with salt and pepper. Pre-heat a large, heavy skillet on a medium-high flame and then add olive oil. Add the chicken in a single layer, (skin-side down, if skin-on) and cook until deep golden brown, about 5-7 minutes. Flip and lightly brown the other side, about 5 minutes. Remove chicken to a plate and set aside.

Add the mushrooms and let them cook on one side without stirring until they're deep brown and caramelized on one side, about 5 minutes. Toss the mushrooms and add leeks to the pan and sauté until softened, about 5 minutes. Add the garlic and cook for 30 seconds more.

Add the wine and let it reduce down by half and then add the lemon zest, thyme, mustard, chicken stock and cream. Return the chicken pieces to the pan and simmer, uncovered, until the chicken is cooked through, 15-25 minutes, depending on thickness. If using skinless chicken, turn pieces in the sauce a few times while cooking to keep it moist). Check the liquid level a few times while cooking and add more stock as necessary. When chicken is cooked through (registers 165ºF on an instant read thermometer), remove it to a platter. Stir in the spinach until just wilted, check seasoning and spoon sauce over chicken.