Toss together salt, thyme, pepper and lemon zest; sprinkle all over chicken legs in bowl. Cover and refrigerate overnight or up to 24 hours.
Preheat oven to 300°F (150°C). Arrange chicken in large Dutch oven in single layer. Pour oil over top, adding enough oil to submerge chicken completely. Cover and bake for 2 to 2 1/2 hours or until chicken is very tender but not falling off the bone. Let stand in oil for 30 minutes. Carefully transfer to foil-lined baking pan fitted with metal rack. Broil on centre rack for about 10 minutes or until skin is crisp. Let stand for 5 minutes.
Meanwhile, pour oil through fine-mesh sieve; discard solids. Whisk together 1/3 cup (75 mL) oil, lemon juice, mustard, salt and pepper; toss with bitter lettuces (reserve remaining oil for another use). Serve with chicken.
Tip: Chicken can be poached and removed from oil up to 2 days in advance, then covered and refrigerated. Bring to room temperature before broiling.