Preheat oven to 400 degrees Fahrenheit. Spray a casserole dish with cooking spray and set aside.
In a large saute pan over medium-high heat saute the fennel with the bay leaves and the zest on one lemon for about 10 minutes, until caramelized and fork-tender.
In a large saute pan over medium-high heat, heat the chicken stock, heavy cream, milk and garlic. Once summering add the potatoes and cook for 15-20 minutes. Until tender but not too soft. Strain the potatoes from the liquid and set aside to cool slightly.
Layer the fennel and potatoes into the casserole dish with a touch of the heavy cream mixture. Alternate layers with fennel, potatoes, cheese, and black truffle. Continue this until the casserole dish is full and you have used all the ingredients. Finish the top layer with more cheese and cover with foil. Place into the oven for 15 minutes.
Remove the casserole from the oven, uncover and top with the remaining cheese. Bake the casserole for an additional 10 minutes.
Set your oven to broil and broil the top of the casserole for 1 minute to lightly caramelize the cheese.