Gratins are one of my favorite things to make. Any vegetable with a bread crumb topping is a delicious side dish for meat or fish and they are fairly simple to make. Great for entertaining and you can assemble ahead of time, ready to pop in the oven when you need it.
This mushroom gratin is slightly more involved than some gratin recipes, but I love the rye bread in the topping and the fontina and leeks are delicious additions.
Mushroom, Leek, and Fontina Gratin
2 oz. (4 Tbs.) unsalted butter; more at room temperature for the dish
1-1/2 lb. medium to large white mushrooms, quartered (8-1/2 cups)
4 Tbs. extra-virgin olive oil
Kosher salt and freshly ground black pepper
1 lb. mixed “wild” mushrooms (such as oyster and shiitake), trimmed and cut into bite-size pieces (7 cups)
1-1/2 cups coarse fresh rye breadcrumbs
4 oz. coarsely grated fontina (1 cup)
3 medium to large leeks, trimmed, white and light-green parts only, cut into 1/4-inch dice (about 2 cups), and rinsed well
1/2 cup dry vermouth
1 cup heavy cream
4 sprigs fresh sage, plus 1 Tbs. chopped fresh sage leaves
Position racks in the top and bottom thirds of the oven and heat the oven to 450°F. Butter a 9x13-inch (or similar) baking dish.
In a large bowl, toss the white mushrooms with 2 Tbs. of the oil, 1/2 tsp. salt, and 1/4 tsp. pepper. Spread the white mushrooms evenly on a large rimmed baking sheet. Toss the “wild” mushrooms with the remaining 2 Tbs. oil, 1/2 tsp. salt, and 1/4 tsp. pepper in the bowl and spread on a second large rimmed baking sheet. Roast, stirring the mushrooms and rotating the baking sheets once, until golden-brown and any liquid has evaporated, about 25 minutes. Reduce the oven temperature to 350°F.
Meanwhile, melt 2 Tbs. of the butter and pour into a medium bowl. Add the breadcrumbs and fontina and toss well.
In a 12-inch skillet, melt the remaining 2 Tbs. butter over medium heat. Add the leeks and 1 tsp. salt and cook, stirring often with a wooden spatula, until golden, 7 to 10 minutes. Add the vermouth and boil, stirring and scraping up any browned bits, until reduced by half, 1 to 2 minutes. Stir in the heavy cream, sage sprigs, 1/2 tsp. salt, and 1/4 tsp. pepper and bring just to a boil over high heat. Lower the heat to medium low and simmer for 5 minutes to infuse. Remove the pan from the heat and discard the sage sprigs.
Add the mushrooms and chopped sage to the leek mixture and toss to coat. Transfer to the prepared dish. Top with the breadcrumb mixture and bake in the top third of the oven until the breadcrumbs are golden-brown, about 30 minutes.
Let rest for 15 minutes before serving.