Preheat oven to 350ºF.
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 purée 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.