Do ahead: Spinach can be blanched and chopped several hours or a day in advance. Cover and refrigerate until needed. Gratin can be fully assembled and 30 minutes before needed, placed in a preheated 375 degree oven to bake, then served.

To stem spinach, as per Julia: If spinach is young and tender, remove the stems at the base of the leaf. If more mature, fold the leaf vertically with its underside up, grasp the leaf in one hand and the stem in the other and and rip it off toward the tip of the leaf, removing the stem and the tough tendrils. Discard any wilted or yellow leaves.

How I wash greens: Fill a large bowl or basin with cold water. Drop in the spinach and swish it around a few times so it deposits any sand and grit before lifting it out of the bowl (leaving any grit/dirt at the bottom of the bowl and not dragging the clean leaves through it) and drop it in a colander. No need to dry it for this recipe, but in others, I either spread it out on towels or run it through a salad spinner.