Potato, Cauliflower, and Onion Bake

The potatoes and onions harmonize with the cauliflower in this recipe, making for a slightly different twist on a classic bistro side.

Serves: 8Hands-on: 15 minutesTotal: 1 hour 20 minutesDifficulty: Medium

Serves: 8


  • 1 Tbsp. olive oil
  • 2 medium sweet onions, diced
  • 2 cloves garlic, minced
  • 3 Tbsp. butter
  • 3 Tbsp. flour
  • 3 cups 2% milk
  • ½ tsp. salt
  • ¼ tsp. pepper
  • ½ tsp. mustard powder
  • ¼ cup shredded Gruyère cheese
  • 2 lbs. russet potatoes
  • 4 cups cauliflower, cut into miniflorets


  • Preheat the oven to 350°F and spray a 9" × 13" casserole dish lightly with nonstick spray.
  • Heat the olive oil in a medium nonstick skillet over medium-low heat. When hot, add the onions and garlic and cook, stirring occasionally, until tender, 10–15 minutes. Do not let the onions brown.
  • While the vegetables are cooking, heat the butter in a medium saucepan over medium heat. When melted, add the flour and stir to combine. Slowly stream in the milk, whisking constantly to avoid lumps. Whisk in the salt, pepper, and mustard powder. Simmer the sauce, whisking often, until thickened, about 3 minutes. Remove the sauce from the heat and whisk in the Gruyère cheese.
  • Peel and slice the potatoes into thin ⅛" thick rounds and spread them evenly along the bottom of the casserole dish. Layer the onion mixture over the potatoes, then spread the cauliflower over the onions. Pour the sauce mixture over the vegetables, cover tightly with foil, and bake until the potatoes are just tender, 45 minutes.
  • Remove the foil from the casserole and bake an additional 15–20 minutes until the top of the casserole is browned and bubbly and the vegetables are completely tender. Allow the casserole to rest for 10 minutes before slicing.