Lay the thighs smooth side down on a chopping board (slice any strings of meat that are preventing them from lying flat). Use the blunt end of your knife to gently pound any thicker parts of the thigh so everything is even.
In a large shallow dish, combine the flour, salt and pepper. One by one, dredge the thighs through the flour, give them a shake, then place to one side.
Add the olive oil to a large pan over medium-high heat. Once hot, add the thighs and fry both sides until golden and cooked right through the centre (4-5 minutes on each side depending on size). Remove and place on a plate to one side, leaving any excess fat in the pan. Work in batches/top up with oil if needed.
Lower the heat to medium, then add the shallots to the leftover fat. Fry until soft and golden, then add the garlic and fry for 10-20 seconds or so (be careful it doesn't burn). Pour in the stock, then stir in 2 tbsp lemon juice, 1 1/2 tbsp dill and 1/2 tsp Dijon mustard.
Take the butter out of the fridge and swiftly stir it into the sauce until it melts and emulsifies into the sauce. Stir in the resting juices from the chicken, then check for seasoning and adjust if needed. If it looks like it's splitting at any point, just lower the heat and whisk it back together. The sauce should have thickened slightly into a light, glossy sauce.
Add the thighs back to the pan to coat in the sauce and warm them back through. Serve with a light zesting of lemon and the rest of the dill. Slice the lemon you used for zesting into 4 wedges (1 per person - optional for extra lemon). Tuck in and enjoy!