From Local Flavors by Deborah Madison, and shared by basketeer Derek Stuart

3 large leeks, white part only
2 fennel bulbs
sea salt & ground pepper
1½ tablespoons unsalted butter
1 bunch scallions (I say use some green garlic instead– K)
¼ c chopped fennel greens
1 teaspoon grated lemon zest
2 large eggs
1½ cups milk or half & half
½ cup grated parmesan or gruyere

1. Preheat oven to 375.  Lightly butter 2-quart gratin dish.  Chop leeks into 1/2-inch pieces & wash, separating the rings.  Let them soak while you trim fennel.  Slice fennel very thinly, including the core. Bring skillet of water to boil, add fennel & pinch of
salt.  Simmer about 2 min until fennel is translucent, drain.
2. Melt butter in wide skillet.  Remove leeks from soaking water and add to pan with fennel.  Season with 1 teaspoon salt and cook for 5 minutes over medium
heat, stirring often.  Add scallions and cook for another 5 minutes.  Add fennel greens and lemon zest, salt, & pepper to taste.  Scrape veggies into prepared dish.
3. Beat eggs and milk together and add 1/2 teaspoon salt plus cheese.  Pour over veggies.  Bake until top is browning, about 40 minutes.  
Let it rest for a few minutes, then serve.

Many thanks to Keeshi + Photo and Twisted String for their photography contributions