vegetable oil, divided • chicken breast or thighs, boneless and skinless, cut into 1/2-inch cubes • salt, divided • garlic • freshly ground black pepper • dried thyme • onion, chopped • carrots, peeled and cutinto 1/4-inch thick slices • celery stalks, cut into 1/4-inch thick slices • dried tarragon • unsalted butter • all-purpose flour •