Set your oven to 400°F (204°C) to preheat.
In a small bowl, blend the mustard, 1 teaspoon lemon zest, oregano, thyme, salt, and 1 tablespoon olive oil to create the rub.
Coat both sides of the chicken thighs evenly with the mustard rub using your fingertips or a brush.
Mix the remaining 2 teaspoons lemon zest, lemon juice, and chicken stock in a bowl or jug and set aside.
Warm the other tablespoon of olive oil in a large oven-safe skillet over medium heat.
Add the smashed garlic cloves and sauté for about a minute until fragrant and edges are browned, then remove the garlic but keep the flavored oil.
Place the chicken thighs skin-side down in the hot oil and cook for 2 to 3 minutes until the skin is nicely browned.
Turn the chicken over, pour in the lemon juice mixture, stir briefly, then transfer the pan to the oven.
Bake for 25 to 35 minutes until the chicken reaches an internal temperature of 175°F.
Take the skillet out of the oven and move the chicken to a plate, loosely covered with foil.
Keep the pan liquid and set the skillet back on medium heat to bring the sauce to a gentle simmer.
Combine butter and flour with your fingers in a small bowl until crumbly.
Whisk this butter-flour mix into the simmering lemon sauce until it thickens and shines.
Nestle the chicken thighs back into the skillet with the sauce before serving.