Add in the drained spinach, artichoke, mustard, garlic, oregano, and cayenne.
Mix and cook on low for 3 hours or high for 1½ hours (I find the 3 hours works better).
Just before serving, add mayo, sour cream, cheeses, and lemon juice. Add crab/lobster, if using. Stir to combine and melt cheeses. Serve warm with crusty bread or crackers.
Recipe by story of a kitchen at https://www.storyofakitchen.com/appetizers/crockpot-spinach-dip-recipe/