Place the potato chunks in a large pot of cold, salted water.
Bring to a boil over high heat, then reduce to a simmer and cook until fork-tender, about 15 minutes.
Drain the potatoes thoroughly and return them to the warm pot.
While the potatoes are cooking, combine the chopped fennel and milk in a small saucepan.
Simmer over medium-low heat until the fennel is very tender, about 15 minutes.
Carefully transfer the hot fennel and milk mixture to a blender.
Add the yogurt, sea salt, and white pepper.
Blend until the mixture is completely smooth and creamy.
Using a potato masher, lightly smash the cooked potatoes in the pot to break them up.
Pour the creamy fennel mixture over the potatoes.
Stir everything together until just combined. Be careful not to overmix; a slightly chunky texture is perfect.
Serve warm.
Don't discard the fennel fronds! Finely chop the delicate, leafy fronds and use them as a fresh, beautiful garnish before serving.
