Remove the spinach from the packaging and thaw it in the microwave in two batches. (Place the spinach on a microwave-safe dish or in a glass pan, cover it with wax paper and heat it for three minutes on high for each batch.) Place the spinach in a colander in the sink to drain.
Thaw the frozen artichoke hearts in the microwave or drain and rinse canned artichoke hearts. Pat the artichokes dry and coarsely chop them.
While the vegetables are in the microwave, chop the onions and garlic. Grease a 9x13 pan with cooking spray. Preheat the oven to 350 degrees F.
Crack the eggs, separating the whites into a medium bowl and the yolks into a small bowl. (You can place one egg white at a time into a measuring cup and then pour that into the bowl for the whites. If you accidentally get some yolk in the egg white, you'll have to replace the egg white.)
Using a hand mixer, beat the egg whites, starting at low speed and increasing to high, until stiff peaks form and don't fall over when you lift the beaters. (It will look fluffy, like meringue.) You can chill the egg whites until they are needed.
In a 2-quart saucepan, melt the butter over medium heat. Add the onions, and after a couple of minutes, add the garlic. Cook until the onions are soft, at least five minutes total.
While the onions and garlic are cooking, whisk the evaporated and 2% milk together. Leaving the egg yolks in a separate bowl, beat the yolks with a fork. Place the milk mixture and egg yolks near the stove.
In the saucepan, stir in the flour, salt and pepper until blended. Gradually stir in the milk mixture, and continue stirring, over heat that's just past medium, until the sauce boils and thickens slightly.
Spoon a small amount of the sauce into the egg yolks and stir to temper the eggs. Slowly pour the egg mixture into the sauce in the pan, stirring quickly to prevent lumps. Stirring constantly, cook until the sauce thickens, but do not let it boil. Remove the sauce from the heat and stir in half of the Parmesan (use ¼ cup.)
Squeeze the spinach dry (I use a clean dish towel that doesn't shed fibers; paper towels also would work.) Add the spinach and artichokes to a large bowl. Pour the sauce over the vegetables and stir to combine. Using a silicone spatula, gently fold in the beaten egg whites until they are incorporated into the mixture.
Transfer the vegetable mixture to the 9x13 pan and smooth the top evenly. Sprinkle on the remaining ¼ cup of Parmesan. Bake uncovered for 25 minutes, or until a knife inserted into the center comes out clean.
Store leftovers covered in the refrigerator for up to four days.
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Notes
Artichokes - Trader Joe's makes frozen artichokes. Birds Eye no longer carries this product.To Make Half: This recipe can be cut in half for an 8x8 pan that will make 8 servings. Bake it for the full amount of time in the recipe.Make-Ahead Options
You can assemble and bake it a couple of days prior to serving. Then reheat it in the microwave or oven.
You can assemble the casserole the day before and refrigerate it without baking. The next day, let it come to room temperature for 30 minutes, then bake. You may need to add a few minutes to the baking time; check for doneness by inserting a knife in the middle and seeing if it comes out clean.
You can start a couple of days ahead of time by thawing and drying the spinach and artichokes, as well as grating the Parmesan. Refrigerate those. The following day, you can make the cream sauce and assemble the casserole. Then you can wait another day to bake it, if you choose.