Delicious and make-ahead-able, this Creamy Mushroom and Gruyere Casserole is perfect for Thanksgiving, Christmas or for entertaining any time of year. It’s also easy enough for family dinners, too!

mushroom leek gruyere casserole in baking pan with spoon

Here in Canada, we celebrated our Thanksgiving back in mid-October, but I’d have to be living under a rock though not to realize that American Thanksgiving is fast approaching. I empathize with those of you scrambling to finalize your menu and get your grocery shopping planned and done! So to that end, I thought I’d offer up a little something – hoping it might be helpful for anyone looking for one more dish to round-out their special meal.

If your family is like my family, there are certain things that will always show up on the Thanksgiving table. In my family, it’s turkey and gravy, of course, my Mom’s make-ahead mashed potato casserole (with cream cheese and sour cream) and traditional bread/sage stuffing. After that, it tends to change up a bit every year. Sometimes we have soup. Sometimes salad. And there’s always a few vegetable dishes of some sort. It seems every year though I’m always looking for just one more dish. I think this mushroom dish fits that bill perfectly! It’s different from the usual, easy to make (and make ahead), re-heats quickly and has flavours that everyone will love

Cook’s Notes

This mushroom side dish features sauteed leeks and a creamy sauce flavoured with a bit of grainy mustard and lemon juice. The lemon juice is the perfect touch of acidity that heightens the flavours and prevents this dish from becoming overly rich. On top, is a crispy topping made from fresh bread crumbs (just throw a couple of slices of bread in a food processor) and some Gruyere cheese. It doesn’t have a ton of cheese. Gruyere has loads of flavour, so a little goes a long way. It’s the perfect complement to the mushrooms.

Of course, it goes without saying that this dish would also be perfect any time of year – for entertaining, or next to simple grilled chicken, pork or beef for any meal at home. It comes together quickly and is sure to become a family favourite.

mushroom leek gruyere casserole in baking pan with spoon

mushroom leek gruyere casserole in baking pan with spoon

Get the Recipe: Creamy Mushroom Leek and Gruyere Casserole

A delicious side dish of mushrooms and leeks with a creamy sauce, flavoured with mustard and lemon juice and topped with a crispy bread crumb and cheese topping. Can be made ahead!
5 stars from 1 rating
Prep Time: 10 minutes
Cook Time: 40 minutes
Total Time: 50 minutes
Yield: 8 servings


  • 5 Tbsp. butter, divided
  • 1 stalk leek, (white and light green parts only), halved lengthwise and then sliced crosswise
  • 1/2 tsp. each salt and pepper
  • 3 8 oz pkgs cremini and/or button mushrooms, sliced
  • 1/3 cup all-purpose flour
  • 1 1/4 cups chicken broth, or sub vegetable broth to keep it vegetarian
  • 1/2 cup whipping cream 35%
  • 2 Tbsp. grainy Dijon mustard
  • 1/2 cup chopped fresh parsley
  • 1 Tbsp. chopped fresh thyme
  • 4 tsp. lemon juice

Gruyère Crumb Topping:

  • 2 cups fresh bread crumbs, slightly stale bread, processed in food processor
  • 3/4 cup grated Gruyère cheese
  • 2 Tbsp. butter, melted
  • 2 Tbsp. chopped fresh parsley


  • In a large skillet, melt 1 Tbsp. of the butter over medium heat. Cook leeks and half each of the salt and pepper, stirring often, until leeks are softened, about 6 minutes. Remove to bowl and set aside.
  • In the same skillet, melt the remaining 3 Tbsp. butter over medium heat. Cook mushrooms with the remaining salt and pepper, stirring occasionally, until mushrooms are just tender and have released their juices, 5 to 8 minutes. Sprinkle with flour and cook, stirring, until light golden, about 2 minutes. Return the leeks to the pan.
  • Whisk in broth, cream and mustard until smooth. Cook, whisking, until thickened, 3 to 5 minutes. Stir in parsley, thyme and lemon juice. Spoon into shallow 8-cup casserole dish. (Make-ahead: Let mushrooms cool to room temperature, then cover and refrigerate for up to 24 hours. To reheat, cover with foil and bake in 375° F. oven until hot and bubbly, 40-50 minutes. Uncover and continue with recipe. You can also mix together the crumbs and cheese of the crumb topping and refrigerate ahead. When ready to broil, stir in parsley and melted butter.)
  • Gruyère Crumb Topping: In bowl, stir together bread crumbs, Gruyère cheese, butter and parsley. Sprinkle over mushroom mixture. Broil until topping is golden, 2 to 4 minutes.
Cuisine: American
Course: Side Dish
Author: Jennifer
Calories: 327kcal, Carbohydrates: 24g, Protein: 8g, Fat: 21g, Saturated Fat: 12g, Cholesterol: 61mg, Sodium: 513mg, Potassium: 125mg, Fiber: 1g, Sugar: 1g, Vitamin A: 745IU, Vitamin C: 6.5mg, Calcium: 198mg, Iron: 2mg
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