Preheat oven to 350 degrees F.
Trim fennel bulbs. Remove root end and remove fennel fronds. Keep a couple of the fennel fronds for garnish, optional. Shave fennel on mandolin very thinly. Place shavings in a large bowl with cold water and gently agitate to remove debris. Drain in colander and set aside.
Trim dark green ends off of leeks, keeping the white and light green parts. Cut leeks in half long way, then cut into half moons. Place leeks in a large bowl with cold water and gently agitate to remove debris and sand. Drain in colander and set aside.
Add 2 cups of white wine and 1 tablespoon of butter to a small sauce pot. Add leeks and gently cook until leeks are tender, about 12 minutes. Drain and set leeks aside and poaching liquid aside separately.
Using the same poaching pan add heavy cream and crushed peppercorns. Bring the cream up to a simmer and remove from heat. Allow to steep while you assemble your gratin.
Prepare baking dish by using remaining 2 tablespoons of butter to coast bottom and sides of baking dish.
Begin by layering shaved fennel on the bottom of the pan. Next, add poached leeks in a scattered layer. Add 1/3 of the dried shallots, freshly ground black pepper and Kosher salt to taste and 1 tablespoon of the cracked fennel. Add 1 cup of the grated Swiss cheese. Repeat this layering process 2-3 more times.
Add the peppercorn cream and 1 cups of the leek poaching liquid together, and pour over layered fennel. Top with the remaining Swiss cheese.
Cover dish with plastic wrap and then aluminum foil. Bake for 45 minutes. Remove wrap and foil. Turn oven up to 400 degrees F and cook for an additional 12-15 minutes until lightly golden brown on top.
Allow to cool for about 20 minutes. Slice into 8 serving pieces. Garnish with chopped parsley and fennel fronds and enjoy!