Preheat the oven to 350 degrees. In a medium bowl, combine the spinach, onions, cream cheese, Cheddar cheese, Monterey Jack cheese, mozzarella cheese, garlic, jalapeno pepper, and seasoned salt. Stir until well blended. Spread into a 1-quart casserole dish and bake for approximately 30 minutes or until the top is nice and bubbly. Serve with sour cream, salsa, and warmed tortilla chips.