Add enough water to a sauce pot so there is about 2 inches of water. Bring it to a boil along with a couple teaspoons of vinegar.
Reduce to a simmer, swirl the water with a spoon and add in the cracked egg, cooking for about 2 minutes. Here is a great video on how to poach an egg.
Melt 1 tablespoon of butter in a sauté pan, add in the spinach and a pinch of salt. Cook until spinach has fully reduced.
Stack the cooked spinach on top of the toast and add the poached egg, with salt and pepper.