In a medium non-stick frying pan, heat oil. Add mushrooms, and garlic if desired. Cook until softened, 4 to 5 minutes, stirring occasionally.
Add spinach and a pinch each of salt and pepper; stir to combine. Cook for a minute until spinach has wilted, then push mixture to one side of the frying pan or remove mixture from the pan and keep it warm.
Place bread in the toaster. While bread is toasting, crack an egg into the frying pan. Cook until the white is set and the yolk is cooked as desired. (A runny yolk makes a delicious “sauce” for this dish, but cook the yolk as you prefer.)
Remove toast from the toaster and set on a plate. Place cheese on toast. Spoon mushroom and spinach mixture over cheese, then add fried egg on top. Sprinkle with salt and pepper, if desired.