Season the chicken with salt and pepper.
Heat a pan or dutch oven over medium heat. Add the oil and onions and cook for 5 minutes, until softened. Sear the chicken on both sides. Then add the apple cider, broth, garlic, and thyme. Cover with a lid and bring to a simmer until the chicken reaches an internal temperature of 160F / 71C.
Mix the flour with an equal amount of water to make a paste. Stir it into the pot in small increments until the liquid thickens to a gravy consistency. Then remove from the heat and set aside.
Make the Potatoes: Place peeled and diced potatoes in a pot of salted water. Boil until tender, about 10 - 15 minutes.
Drain the cooked potatoes and mash them with butter, cream, salt, and pepper until smooth and creamy.
Make the Spinach: In a large skillet over medium heat, add the oil. Add the chopped shallot (or onion) and sauté for 2 - 3 minutes until softened. Add the garlic and cook for another minute.
Add the spinach to the pan and cook, stirring occasionally, until wilted (about 2 - 3 minutes).
Reduce the heat to low. Stir in the heavy cream and simmer until the cream reduces to a creamy consistency.
Stir in the Parmesan cheese until melted and well combined. Cook for another minute or two, until the sauce thickens slightly. Season with salt, pepper, and a pinch of nutmeg if desired.
Mix the sparkling water with lemon. Serve over ice if desired.
Serve the chicken with mashed potatoes, spinach, and lemon sparkling water.