Preheat oven to 350 degrees. Sprinkle the garlic powder, seasoning salt, and pepper on both sides of the chicken. Bake for 16 minutes.
Meanwhile, make the spinach. Over a medium-high heat, add 2 TBSP Olive oil to a deep skillet. Add the onions and sauté 2-3 minutes. Add the spinach and garlic. Spinach will shrink considerably. When cooked through, add the cream cheese. Continue to cook until the cream cheese is melted. Remove from heat.
For the mushrooms, melt butter and add 2 TBSP Olive oil to a skillet. Put the mushrooms in and cook until browned all over. Don't season with salt until they are browned or they will never brown. Deglaze the pan with the wine. Add in the garlic and season with salt and pepper. Cook until the wine is almost cooked out.
Mix the mushrooms in with the spinach and stir together.
Remove chicken from the oven and flip over. Cover each chicken breast with the spinach and mushroom mixture. Put 2-3 slices of mozzarella cheese over each chicken breast. Return to the oven and cook for an additional 12-14 minutes.