Heat a large cast iron skillet over medium heat. Melt butter and saute onion for 5 minutes or until tender. Add in spinach and stir for 1 minute to wilt. Transfer cooked onions and spinach to cutting board to cool slightly. Chop into small pieces.
Place cream cheese into hot skillet over low heat and begin to stir to melt. After about 2 minutes, cream cheese should be very soft and smooth. Stir in chopped onions and spinach, artichoke hearts, yogurt, mayonnaise, shrimp, 1 cup parmesan cheese, salt and pepper. Stir to combine. Spread evenly in pan and sprinkle with remaining 1/4 cup of parmesan cheese.
Bake 20 minutes or until hot and bubbly. Remove from oven and serve immediately with bread or crackers.