To make the green goddess dressing: Simply puree the ingredients together in a blender, until light green and creamy. Cover and stick the dressing in the fridge to allow the flavors to come together.
To prepare the soft-shell crabs: Pour the buttermilk in a shallow bowl large enough to hold all the crabs. Season the buttermilk with crab boil seasoning, salt, and pepper. Lay the crabs in the buttermilk mixture and marinate, covered, in the refrigerator for 1 hour; soaking the crabs in buttermilk helps plump them up when cooked. Remove the crabs from the milk bath and let the excess drip off.
Pour 2 inches of oil in a heavy frying pan or large pot and heat to 350 degrees F.
Put the flour in a pie dish or plate and season with a fair amount of salt and pepper. Dredge the crabs in the seasoned flour to coat, shaking off the excess. Lay the crabs in the hot oil in a single layer without crowding; you may have to fry them in batches. Be careful, the crabs have a tendency to pop and spatter. Cook the crabs for about 3 minutes on each side, turning once, until golden brown. Drain on paper towels.
To assemble the salad: combine the watercress, lettuce, and onion slices in a mixing bowl. Pour in 1/2 cup of the green goddess dressing, tossing to coat. Arrange the salad on a serving platter, put the fried crabs on top, and grate the hard boiled egg over the whole thing. Garnish with lemon wedges and serve immediately.