Step 1Toss together the butter lettuce, romaine and torn sorrel in a glass bowl. Cover with damp paper towels and refrigerate until ready to serve.
Step 2While the greens are chilling, place the mayonnaise, sour cream, garlic, green onion, chopped sorrel, anchovies, lemon juice, dill weed and milk in a blender and blend until smooth. Add salt and pepper to taste, then blend to incorporate them.
Step 3Spoon the dressing onto chilled greens and toss gently.