2. Grease twelve 1/2 cup oven proof ramekins with butter.
3. Melt the 2 tablespoons of butter in a small skillet over medium heat. Cook onion until soft and reserve.
4. Bring a medium pot of water to a boil. Add the carrots and cook until tender, about 10 minutes. Drain and transfer to a food processor or blender.
5. Add the cooked onion to the carrots and purée until smooth. Add the eggs, yolks, salt, pepper, nutmeg, sugar, zest, and juice and process to combine. Pour in the hot milk and melted butter. Process to combine.
6. Pour into the prepared ramekins and transfer to a roasting pan. Pour hot water halfway up the sides of the ramekins. Bake about 40 minutes, until a knife inserted in the center comes out clean.
7. Cool on a rack then invert directly on dinner plate.