Set your oven to 400°F (204°C) to preheat.
In a small bowl, blend whole grain mustard, 1 teaspoon lemon zest, oregano, thyme, salt, and 1 tablespoon olive oil. Thoroughly coat both sides of the chicken thighs with this mixture.
Combine the remaining 2 teaspoons lemon zest, lemon juice, and chicken stock in a separate container and set aside.
Warm the remaining tablespoon of olive oil in an oven-safe skillet over medium heat.
Add the smashed garlic to the oil and sauté for about a minute until fragrant and lightly browned, then discard the garlic cloves but keep the infused oil.
Place the chicken thighs skin-side down into the skillet and cook for 2 to 3 minutes until the skin is golden and crisp.
Turn the chicken over, pour in the lemon stock mixture, stir gently, then transfer the skillet to the oven.
Bake the chicken for 25 to 35 minutes until a meat thermometer inserted into the thickest part reads 175°F.
Remove the pan from the oven and move the chicken to a plate, covering loosely with foil to keep warm.
Return the pan with the remaining liquid to the stovetop over medium heat and bring to a gentle simmer.
Mix the butter and flour together with your fingers forming a crumbly paste.
Whisk this butter-flour paste into the simmering lemon sauce until it thickens and turns glossy.
Nestle the chicken thighs back into the pan with the sauce and serve immediately.