6cupsfrozenriced cauliflower (1-1/2 packages - see notes)
Salt and freshly ground black pepper
Preheat the oven to 375°F and coat a 1-quart baking dish with nonstick spray.
Heat 2 tablespoons of the butter in a large frying pan over medium heat. Add the onion and sauté until soft and barely golden, 4 to 5 minutes. Add the garlic and sauté until fragrant, 1 minute longer.
Sprinkle the mixture with the flour and combine well. Continue cooking for 1 minute, then slowly whisk in the warm milk, stirring constantly. Bring the mixture to a simmer and continue cooking until smooth and slightly thickened, 2 to 3 minutes.
Add the riced cauliflower, 2 cups at a time, combining well after each addition. Continue cooking, stirring often, until the cauliflower is tender and heated through, 5 minutes.
Season to taste with salt and pepper, then transfer the cauliflower to the prepared baking dish.
Melt the remaining tablespoon of butter in the pan, add the panko crumbs and combine.
Sprinkle the buttered crumbs over the cauliflower and bake until the gratin is bubbly and lightly browned on top, 12 to 15 minutes. Serve immediately.
Tips for Making This Recipe
Recipe Variation: Gratin of Riced Cauliflower and Greens
To make this variation, follow the directions above, but cut the quantity of riced cauliflower back to 4 cups (1 package frozen). Add 1/2 pound of chopped greens (spinach, baby kale or escarole) that have been sautéed until tender. If desired, replace the buttered panko topping with a handful of shredded cheddar cheese. Bake as directed above.