Make the rub for the chicken thighs by combining the whole grain mustard, 1 teaspoon of the lemon zest, oregano, basil, and 1 tablespoon of the olive oil in a small bowl. Coat the chicken on both sides with the mustard rub. Season both sides of the chicken with sea salt and freshly cracked pepper, to taste.
In a measuring cup, combine the remaining lemon zest (2 teaspoons), lemon juice, and the chicken broth. Set aside.
Heat the remaining tablespoon of oil in a large OVEN-SAFE skillet or Dutch oven over medium heat. Add the chicken into the hot oil and cook for 2 to 3 minutes or until it has turned golden brown. Flip the chicken then add the minced garlic to the pan and cook, stirring constantly for 30 seconds. Add the lemon juice mixture and stir, making sure to scrape any of the browned bits off the bottom of the pan. Place the pan into the oven and bake 20 minutes, or until chicken is cooked through.
Remove the pan from the oven then transfer the chicken to a plate and cover it with an aluminum foil tent. Place the pan over medium heat and bring the liquid to a gentle boil.
In a small bowl, use your fingers to mix the butter and flour together, making a dry, crumbly paste. Whisk the flour and butter into the sauce. The sauce will thicken slightly and look glossy. Taste and season with sea salt and freshly cracked pepper, if needed. Place the chicken and its juices back into the pan and coat with sauce. Serve. Enjoy.