Preheat oven to 450˚F.
On a baking sheet, place mushrooms caps stem side up, drizzle with EVOO and season with salt and pepper. Roast 10-12 minutes, until tender. Remove from oven and set aside.
While the mushrooms are roasting, to a food processor add garlic, onion, EVOO, salt, pepper and nutmeg, and pulse to chop. Add artichokes and cheese, and pulse again. Add spinach, egg yolks, pulse again to combine, then add in nuts and heavy cream.
Fill mushroom caps with spinach-artichoke mixture and bake another 10-12 minutes, until filling is cooked through.