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Published December 23, 2022. This post may contain affiliate links. Please read my disclosure policy.
This delicious sweet potato au gratin recipe is loaded with sliced garnet sweet potatoes that are simmered in a creamy sauce and topped with crispy breadcrumbs. This is the perfect side dish.
Sweet Potato Au Gratin
Sweet potato au gratin is a classic au gratin-style dish but utilizes slices of sweet potatoes coated in a mornay sauce and topped off with breadcrumbs before baking. The word au gratin has a couple of meanings. It can translate to “to grate” or “with a crust.”
Applying au gratin to any food or recipe means finishing by sprinkling breadcrumbs over the top. You can essential au gratin just about anything. In addition to the breadcrumbs, you can add some extra cheese and herbs to enhance the flavor.
Ingredients and Substitutions
- Potatoes – I like to use red garnet potatoes. In addition, I usually add in 1 russet potato.
- Onion – You can use a white, yellow, or sweet onion.
- Flour – All-purpose flour is used to help make a roux for the bechamel sauce.
- Butter – Always use unsalted butter in cooking and baking to control the salt content.
- Cheese – I like to combine gruyere and parmesan to make the final mornay sauce.
- Breadcrumbs – You will need some breadcrumbs to top off the sweet potato au gratin.
- Milk – I prefer to use whole milk, but you can substitute it with 1%, 2%, or skim milk.
- Herbs – For garnish, I used fresh chopped thyme. However, you can use rosemary or parsley.
How to Make Sweet Potato Au Gratin
Add butter to a large rondeau or pot over low to medium heat.
Once melted, add onions and cook for 10 to 15 minutes or until decently browned.
While the onions are browning, you can slice up the potatoes.
Stir the flour into the onion until completely combined to make a roux.
Pour in the milk and turn the heat high while vigorously whisking to ensure the roux is incorporated into the milk.
The sauce will become very thick in a short time once it starts to bubble, and it should easily coat the back of a spoon.
Turn the heat down to low and whisk the gruyere, parmesan, butter, salt, and pepper until mixed in. Be sure to taste the sauce, as it should be well seasoned.
Add in the sweet potatoes and russet potatoes and cook over low heat for 12 to 15 minutes to start the cooking process of the potatoes.
Transfer the mixture to a 13×9 or 12×8 casserole dish and smooth it out on top by gently pressing down.
In a small bowl, mix 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.
Make-Ahead and Storage
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.
Chef Notes + Tips
- 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.
More Potato Recipes
Sweet Potato Au Gratin Recipe
- Preheat the oven to 350°.
- 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 13×9 or 12×8 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.