This broccoli and cauliflower gratin recipe only takes about 30 minutes from start to finish! Cauliflower is my go-to vegetable when I need something to replace starchy potatoes. It helps cut down on carbs and create a healthier version of au gratin potatoes. But the cheese sauce is the real star of the show– so creamy, dreamy, and delicious. This broccoli cauliflower casserole tastes just like my favorite holiday side, but healthier!
How to Store and Reheat
Store leftover broccoli and cauliflower gratin tightly wrapped with plastic wrap or aluminum foil in the refrigerator for up to 4 days. Reheat in a 350°F oven for 20-30 minutes, or until warmed through. I do not recommend freezing this dish, as the texture changes too much once thawed.
Serving Suggestions
I like to serve this broccoli cauliflower casserole alongside my favorite holiday mains, like Thanksgiving turkey, brown sugar glazed ham, or beef tenderloin! Add some holiday sides and you’ve got the perfect dinner!
How to Make Broccoli and Cauliflower Gratin Step by Step
Prep: Preheat your oven to 425°F and set the oven rack to the middle position. Spray a 2-quart (11×7-inch) casserole dish with nonstick cooking spray. Make the Topping: Tear 3 slices of sandwich bread into 4 pieces. Place the bread, 2 tablespoons of unsalted butter, ¼ teaspoon of table salt, and ⅛ teaspoon of ground black pepper in a food processor fitted with a metal blade. Process until coarse crumbs form, about ten 1-second pulses. Set aside. Chop the Veggies: Trim 1 head each of cauliflower and broccoli into ¾-inch florets. If using broccoli stalks, use only ½-inch of stalk next to the floret. Boil the Veggies: Fill a Dutch oven or stockpot two-thirds full with water. Add 1 tablespoon of table salt and bring the water to a boil. Stir the boiling water and add the cauliflower and broccoli. Cook until the vegetables have a slight bite, 3-4 minutes, and drain. Cook the Onion and Garlic: Heat a large skillet over medium heat. Add 2 tablespoons of unsalted butter and cook just until foaming subsides. Add ¼ of a diced yellow onion and cook until softened and translucent, 2-3 minutes, then add 1 minced clove of garlic and cook for 30 seconds. Make the Roux: Sprinkle 1½ tablespoons of all-purpose flour over the onion/garlic mixture; stir and cook for about 1 minute. Add the Liquids: Reduce the heat to medium-low and whisk in 1¼ cups of heavy cream and ¼ cup of low-sodium chicken broth (or wine). While stirring, bring to a boil. Season the Sauce: Reduce the heat to low and add 2 pinches of freshly grated nutmeg, 1 teaspoon of dry mustard powder, 1 teaspoon of crushed red pepper flakes, and ⅛ teaspoon of freshly ground black pepper. Cook (and stir) just until the mixture has thickened a little. Add salt to taste if needed. Add the Cheese: Add ½ cup of freshly grated Parmesan cheese and 1 cup of freshly shredded sharp cheddar cheese and stir until cheese is melted. Stir in the Veggies: Remove from the heat and add the cauliflower and broccoli; stir gently to coat the vegetables. Bake the Casserole: Transfer the cauliflower/broccoli mixture to the prepared dish and sprinkle with the breadcrumb topping. Bake at 425°F for 15-20 minutes or until the sauce bubbles around the edges and the top is golden brown. Serve immediately.
