Make the Green Goddess Dressing: In a blender, combine the watercress, parsley, chives, dill, tarragon, lemon juice, garlic, anchovies, and mayonnaise. Blend until smooth. With the motor running, slowly add olive oil. If the dressing is too thick, thin with milk 1 tablespoon at a time until pourable. Season with salt and pepper to taste. Set aside.
Cook the Halibut: Pat halibut fillets dry and season with salt. Heat a sauté pan over medium-high heat and add the oil. Press halibut into the hot pan and sear undisturbed until golden, about 3 minutes. Flip and cook just until medium in the center. Remove from heat and let rest.
Blanch the Asparagus: Bring a small pot of water to a boil. Add asparagus and blanch until bright green and just tender, about 2 minutes. Transfer to an ice bath to stop cooking. Drain and set aside.
Assemble the Salad: Spread a generous spoonful of dressing onto each serving plate. Top with 1 cup of butter lettuce and a halibut fillet. Slice a hard-boiled egg and place it on the plate, then add asparagus, snap peas, and shaved radishes. Drizzle with more dressing if desired.
