How to Make It
In a saucepan, combine the anchovies, garlic and oil. Simmer over moderately low heat until the garlic is very soft but not colored, about 30 minutes. Transfer the mixture to a blender and let cool for 10 minutes. Add the butter and lemon juice and puree until the bagna cauda is smooth.
Meanwhile, bring a large saucepan of water to a boil. Add the asparagus and cook until crisp-tender, about 3 minutes. Using a slotted spoon, transfer the asparagus to a plate and let cool. Add the shelled fava beans to the boiling water and cook for 5 minutes. Drain and let cool under cold running water. Pat dry; if using favas, peel off the beans' tough outer skins.
Mound the watercress on a large platter. Arrange the fennel, radishes, carrots, eggs, asparagus and fava beans on top in separate piles and drizzle with some of the bagna cauda. Pour the remaining bagna cauda into a small bowl and serve with the vegetable platter.