In a skillet, cook bacon without oil until crisp. Drain on paper towels. Then cut each bacon slice in half.
Make onion dip. In a bowl, beat cream cheese and stir in onion and Parmesan cheese. Add salt and black pepper to taste. Mix well.
Cut the bread horizontally into half. On the bottom of the bread, spread the onion dip, put spinach, tomatoes, bacon and cheddar cheese slices. Put the top of the bread to close sandwich.
In a large skillet, melt 1/2 tablespoon of butter over medium heat, place the sandwich, bottom side down, into the skillet and cook until the bottom is golden brown.
Take the sandwich out and place on a cutting board or plate. Melt 1/2 tablespoon of butter in the same skillet, return the sandwich, top side down, into the skillet. Cook until golden brown and cheese melts
Take the sandwich out and place on a cutting board. Cut the sandwich into half and serve.