Make several deep slashes into the chicken legs and then season with salt and pepper.  (For a diabetic friendly dish, remove the skin first) Heat the butter and olive oil together in a large skillet with a lid, until foaming.   Add the chicken legs and brown them all over, turning them frequently.  Add the garlic, lemon zest and thyme.   Whisk together the flour stock mustard and lemon juice.  Pour this around the chicken legs in the pan.   Bring to the boil, then reduce to a slow simmer and cover.   Simmer, covered for 25 to 30 minutes until tender.  Place the drumsticks on a serving platter.  Taste and adjust the sauce for seasoning.  Pour this gravy over the chicken and serve.