Crowd-pleasing Cauliflower Au Gratin is an easy, cheesy side that's the perfect combination of healthy veggies and homey comfort. Make the cheese sauce in advance or use frozen florets to save yourself prep time.
2mediumheads cauliflowerdivided into florets, about 12 cups
1mediumyellow or sweet oniondiced
4tbspall purpose flour
¼tspnutmegfreshly grated, if available
2ccheesegruyere and cheddar
Heat the oven to 375°F. Lightly mist a 2-quart casserole dish with cooking spray, or grease with a pat of butter.
Steam cauliflower the cauliflower using your preferred method until the florets are crisp-tender, but not soft. See notes.
Heat a 2-quart saucepan over medium heat. Add the butter and heat until it melts, then foams. Add the onion and cook until softened and translucent, 5-6 minutes, stirring occasionally. Add the garlic and cook 1 minute more, or until fragrant, stirring constantly.
Sprinkle the flour over the onion and garlic mixture. Work in with a rubber spatula until it forms a paste-like consistency.
Gradually pour in the milk, whisking constantly to avoid clumping. Bring to a steady simmer and cook until the sauce is slightly thickened and easily coats the spatula, about 5 minutes.
Whisk in the salt, pepper and nutmeg. Reduce the heat to low, then add the cheeses in small batches (about ½ cup), whisking after each addition until very smooth.
Arrange the steamed cauliflower in the prepared casserole dish. Pour the sauce over the florets, then gently toss evenly coat the florets.
Combine the breadcrumbs and butter in a small bowl and stir until the breadcrumbs are evenly moistened. Sprinkle the mixture evenly over the casserole dish.
Bake uncovered 25-30 minutes, until bubbling and golden brown.
Rest five minutes, then garnish with parsley and freshly grated nutmeg (if available). Serve hot or warm.
Make Ahead: the sauce can be prepared up to 24 hours in advance. The casserole can be cooked through in the morning and kept covered in the fridge. Gently reheat in a 350°F oven for 15 minutes.
Steamed Cauliflower Methods
Pour ½ cup water into the inner pot. Position a trivet or all metal steamer basket in the pot. Place the florets into the steamer basket, then sprinkle with ½ tsp kosher salt.
Secure the lid and ensure the pressure valve is set to Sealing. Program to HIGH pressure 3 minutes for crisp tender florets. It will take approximately 10 minutes for the pot to come to pressure.
Once the timer goes off, press the Cancel button to turn off the Instant Pot. Then perform a manual pressure release by turning the venting knob from Sealing to Venting. Once the float valve has dropped, open the lid away from your body to allow the steam to escape. Carefully remove the steamer basket with oven mitts, or remove the florets with a slotted spoon.
Place the florets into a microwave safe bowl, then sprinkle with ½ tsp salt. Add the water, then cover the bowl with a plate.
Microwave 5 minutes for crisp tender florets. Strain the cauliflower from the steaming water.
Fill a pot large enough to accommodate a steamer basket with 1.5-2” water. Position the steamer basket in the pot, then bring the water to a simmer over high heat.
Place the florets into the steamer basket, then sprinkle with ½ tsp kosher salt. Reduce the heat to medium low to maintain a steady simmer, then cover.
Steam 5-6 minutes for crisp tender florets. Remove the steamer basket from the pot.