Preheat oven to 350F.
In a medium saucepot, place sweet potatoes in cold water, and bring to a boil. Cook until fork-tender, 10-12 minutes. Once tender, put into a food processor or blender and pure until smooth. Set aside.
Toast the bread cubes in the oven until golden brown, about 6-8 minutes.
In a large skillet over medium-high heat, sweat the leeks, fennel and carrots in 2 tablespoons of butter and the olive oil until softened and set aside.
Combine the eggs, cream and toasted bread. Drain the raisins and add to custard along with the cooked vegetables. Season well with salt and pepper.
In a buttered glass or ceramic dish, spread the sweet potato pure evenly over the bottom then top with the bread mixture. Top the bread with a few pads of butter and bake until bread is golden brown and pure is warmed through, about 30 minutes.