Confited garlic, slowly cooked in oil, flavors the cream sauce for this excellent side dish. The spinach is tossed with the cream, then topped with cheesy breadcrumbs before baking. Slideshow: More Gratin Recipes
In a small saucepan, cover the garlic cloves with water and bring to a boil. Drain and pat dry. Wipe out the saucepan. Add the garlic and canola oil and cook over very low heat until the garlic is softened and golden, about 30 minutes. Drain in a sieve, reserving the oil for another use. Transfer the garlic to a blender.
In a small bowl, whisk 2 tablespoons of the cream with the arrowroot. In a saucepan, bring the remaining cream to a simmer. Whisk in the arrowroot mixture and cook over moderate heat, whisking, until the cream is thickened, about 2 minutes. Stir in the nutmeg and season with salt. Add the cream sauce to the blender and puree with the garlic until smooth.
Preheat the oven to 350°. Lightly oil a 1 1/2-quart baking dish. Working in batches, blanch the spinach in a large pot of salted boiling water just until wilted, 1 minute. Using tongs, transfer to a colander. Squeeze out as much water from the spinach as possible. Using paper towels, blot any excess water. Coarsely chop the spinach and transfer to a large bowl (you should have 6 packed cups). Add the cream sauce and mix well. Season with salt. Spread the spinach in the prepared dish and top with the breadcrumbs and cheese. Bake for about 30 minutes, until the cheese is golden and the sauce is bubbling. Let stand for 10 minutes before serving.
The cream sauce and blanched spinach can be refrigerated separately overnight.