Preheat oven to 400°F. Parboil potatoes 5 minutes in boiling salted water and drain.
Combine spinach, artichoke hearts, ricotta, garlic, salt, pepper, egg yolks and half the cheese in a bowl.
Melt butter in saucepot over medium-high heat and whisk in flour. Cook 1-2 minutes then add milk, season with salt, pepper and nutmeg and thicken sauce to coat a spoon.
Pour half the sauce into a casserole dish and top with a layer of the potatoes, then half the spinach-artichoke mixture. Continue layering the casserole with more potatoes, the remaining spinach-artichoke, the rest of the potatoes and top with the remaining sauce and cheese.