Bring a small amount of water to a simmer in a pot and set a steamer basket over it.
Using a sharp paring knife, peel the stems on the artichokes and remove the tougher outer leaves; remove the inner leaves by hand, then cut away the center chokes. Place the artichokes in the steamer basket, cover and cook until tender when pierced, about 30 minutes, adding hot water if needed during cooking. Roughly chop the artichokes into 1/2-inch pieces and combine them with the cannellini beans and leeks in a large bowl.
Preheat the oven to 375 degrees F.
In a separate bowl, whisk together the Parmigiano-Reggiano, heavy cream and creme fraiche. Pour into the artichoke mixture and toss well. The mixture should be creamy and smooth; if too thick, add more cream to thin. Sprinkle with salt and pepper.
Pour the mixture into an 8-inch casserole dish. In a small bowl, combine the breadcrumbs, thyme leaves and lemon zest; sprinkle over the top of the casserole.
Bake the gratin until the mixture is bubbly and the crust is golden brown, about 30 minutes. Finish with a sprinkle of fleur de sel.