Combine mustard and vinegar in a blender and blend at medium for 15 seconds to combine. With blender running, add ½ cup oil in a slow stream, making sure to add it slowly enough to develop a creamy emulsion.
Transfer mixture to a bowl and whisk in the remaining oil. Refrigerate for up to 2 weeks. Yield: 2-½ cups salad dressing.
Carefully core each lettuce head and loosen leaves but keep structure of head intact, as it will be reassembled on the plate. If using a leaf lettuce, tear leaves into small pieces. Plunge lettuce into cold water to clean it, then gently spin in a salad spinner until very dry.
Transfer lettuce to a large mixing bowl and season with salt and freshly ground black pepper. Scatter herbs and shallot and drizzle vinaigrette and lemon juice over everything. Toss it gently by hand to dress, and serve.
