Keep the mornay sauce hot over low heat in a large rondeau or pot.
Add the sweet potatoes and russet potatoes to the pot with mornay and cook over low heat for 12 to 15 minutes just to start the cooking process of the potatoes. Gently stir the mixture every 2 to 3 minutes using a rubber spatula or spoon.
Transfer the mixture to a 13x9 or 12x8 casserole dish and smooth it out on top by gently pressing down.
In a small bowl, mix together the breadcrumbs and olive oil until mixed.
Evenly sprinkle the breadcrumbs all over the sweet potato mixture until covered.
Bake in the oven at 350° for 30 to 35 minutes or until the breadcrumbs are browned and the potatoes are tender. Do not overcook, or the sauce will break.
Remove from the oven and let it rest for about 10 minutes. Garnish with the option of fresh chopped thyme, rosemary, or parsley.
Notes
Make-Ahead: You can make this recipe up to 1 day ahead. Reheat for 30 minutes before serving.How to Store: I like to store these in the refrigerator covered in plastic for up to 4 days. I also cover and freeze them for up to 2 months. Thaw them for 1 day in the refrigerator before reheating them.How to Reheat: To reheat them, add your desired portion to a pan, cover with foil and bake in the oven at 375° for 20-25 minutes or until hot. In addition, you can add them to the microwave and heat them until hot.You can use any bread of sweet potato or yam in this recipe.The creamy sauce will break if you bake the potatoes for too long. It will still taste good, but it will become very watery. Unfortunately, you can do nothing to emulsify the sauce at this stage.Add 2 teaspoons of chopped thyme, or rosemary, to the mornay sauce while seasonings it if you want some fresh herb flavor incorporated into the potatoes.