Season the chicken with salt and ground black pepper, on both sides. Heat up a skillet with 1 tablespoon butter. Pan-sear the chicken thighs until both sides are nicely browned, with crispy edges. Remove fromthe skillet and set aside.
On the same skillet, add the remaining butter and saute the garlic until aromatic. Add the chicken broth, heavy whipping cream and bring it to a light simmer. Add the chicken back into the skillet, follow by the Parmesan cheese, lemon juice, mirin (if using), and salt.
Add the tomatoes, spinach and parsley. As soon as the spinach wilts, turn off heat and serve immediately.
Notes
Serve the chicken with plain spaghetti, penne or rotini. Just ladle the chicken and sauce over the pasta.