Make the rub for the chicken thighs by combining 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.
Combine the remaining lemon zest (2 teaspoons), lemon juice, and the chicken broth in a measuring cup. Set aside.
Heat the remaining tablespoon of oil in a Dutch oven or a large OVENPROOF skillet 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, 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 in the oven and bake for 20 minutes, or until the chicken is cooked.
Remove the Dutch oven from the oven, transfer the chicken to a plate, and cover it with a foil tent.
Place the Dutch oven over medium heat and bring the liquid to a gentle boil.
Use your fingers to mix the butter and flour, making a dry, crumbly paste in a small bowl.
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 it with the sauce. Serve. Enjoy.