Heat butter in a large, heavy pot over medium-high heat; cook and stir onions until they become translucent, about 5 minutes. Reduce heat to medium.

Cook, stirring constantly, until onions are soft and browned, at least 30 minutes. Stir in garlic and cook until fragrant, about 1 minute. Stir sherry in with onion mixture and scrape bottom of pot to dissolve small bits of browned food from the pot. Add the beef broth. Season to taste with sea salt; stir in thyme and bay leaf. Cook for 30 minutes, stirring occasionally.

About 5 minutes before serving, set oven rack about 10 inches from the heat source and preheat the oven’s broiler. Arrange Protein bread slices on a baking sheet (only make the amount of bread that you will be serving at the moment).

Spread butter on the protein sparing bread slices. Top with the Gruyère cheese, then a sprinkling of Parmesan on each piece. Broil Protein bread slices until toasted, about 1 minute.

Fill soup crocks 3/4 full of onion soup and float a protein bread slice in each bowl. Best served fresh.

Store extra soup without bread added to it in an airtight container in the fridge for up to 6 days. Can be frozen for up to a month. Protein Sparing Bread can also be frozen before broiling.