In a saute pan, on medium-high heat, add the bacon and lay it out thin. Cook the bacon until golden and crispy. Turn the bacon over to the other side and cook until all sides are crispy. Remove the bacon and place on a paper towel to drain. Slice the tomatoes, cheese and avocados. Arrange sandwich with toasted bread, mayo, cheese (optional), avocado slices, tomato slices spinach and bacon. You can cook the sandwich in a panini maker or serve cold.