Place a large cast iron skillet over medium high heat. Once the pan gets hot, put the butter in the pan.
Add the spinach, red bell pepper, and Canadian bacon to the pan. Give it a stir.
While the veggie mixture is cooking, get out a second bowl. In this bowl, combine the heavy cream, egg whites, and salt. Whisk until frothy. Add the cheese.
Once the veggies are soft, add the egg mixture to the pan and stir. Let it sit for about one minute or until the outside edges of the eggs start to turn white.
Put the whole pan in the oven for 10-15 minutes (or until the frittata is mostly firm with a little bit of jiggle still in the middle).