Saute onion in butter and olive oil for 5 minutes or until tender. Add garlic and cook 1 minute and remove from heat.
Stir cream cheese into onions until it melts and blends. Stir in spinach, milk, sour cream, mayonnaise and mustard. Season with salt & pepper.
Mix egg with pimiento or peppers and add 3/4 of each cheese. Mix a spoonful of the warm spinach mixture into the cheese mix to temper the egg. Now add the cheese mix to the spinach in the skillet, stir to combine, and transfer it to a lightly greased baking dish. Refrigerate for up to 24 hours before baking if desired.
Top with reserved cheese. Bake at 350 for 35-45 minutes