Heat a large skillet over medium-high heat for 2 minutes.
Add the onions and cook for 15 minutes, stirring occasionally, until most of the moisture has evaporated from the onions. Add a splash of water and scrape up any stuck bits from the bottom of your skillet. Lower heat to medium and continue to cook for an additional 20 minutes, stirring often and adding a splash of water every time the onions stick or the bottom of the skillet gets brown. After 20 minutes, your onions should be nice and golden brown in color. If your onions aren’t quite golden after 20 minutes, continue to cook an additional 5 - 10 minutes, stirring often, and you should be good to go.
Drain the cashews (discard soaking water) and place them into your blender, along with the non-dairy milk, lemon juice, garlic, nutritional yeast, salt, black pepper and nutmeg. Blend until creamy and smooth then transfer to a large mixing bowl. Add the cooked onions, chopped artichoke hearts and chopped spinach and stir well.
Pour the mixture into your baking dish.