Grease a 9x9 casserole dish with butter and dust with parmesan cheese, set aside. Preheat the oven to 375F.
In a small pot melt the butter, whisk in the flour and cook for a few minutes until the flour is browned and it smells toasty. Slowly pour in the milk while whisking to reduce clumps, add the mustard, pepper and salt and cook till the mixture has thickened. Taste and adjust seasoning as needed, set aside to cool.
In a large pan sauté the the onion with a pinch of salt until softened, add in the spinach tearing it up as you go, cook until wilted. Scrape the mixture into a mesh sieve and push out any liquid. You are hoping to end up with about a cup of spinach mixture, add the peas for a total of 1 ¼ cups of vegetables.
Puree the veggies in a food processor with the sour cream, sherry and egg yolks until smooth. Transfer the mixture to a large bowl, the the béchamel and mix well.
Whip the egg whites in a stand mixer with the salt and cream of tartar until stiff peaks. Fold the egg whites into the spinach mixture a 1/3rd at a time, I like to use a whisk to start the processes and finish with a rubber spatula. Leave no trails of white.
Gently pour the mixture into the prepared pan placing the bowl as close to the bottom to prevent knocking out any air, gently smooth the surface.
Bake at 375 F for 30-35 minutes until a toothpick comes out and it has risen 1-2 inches. Eat immediately!