in a medium skillet melt the butter until it comes to a foam

add the finely chopped onion to the butter and cook until translucent/tender (if your using mushrooms too, saute them with the onions)

meanwhile, whisk the eggs, milk, and seasonings in a bowl

pour the egg mix in skillet with cooked onions. reduce to medium-low heat, do not scramble eggs, let it cook a bottom layer

when the eggs form a cooked layer, gently push the edges in and tilt the pan so that the uncooked egg fills that area. do this around all the edges

after most of the excess egg has been sifted and cooked, flip the cooked egg (frittata) without breaking as much as possible.

place the sliced cheese over the egg and let it cook for 2-3 minutes. top with chopped parsely

toast the slices of bread

spread mayo on insides of both slices of bread (optional)

transfer frittata (cooked egg) to the bread.

top with spinach, cooked bacon, tomato, and lettuce

close sandwhich with 2nd slice of bread, cut sandwhich in half and enjoy!