Turn your Instant Pot to the SAUTE function. Add the butter to the pot and allow it to melt. Then, add the onion.
Stir the onions in the pot for about 10-15 minutes until they start to really whither and brown. You can add extra butter if you think it’s necessary. Once the onions have started to brown, add 1 tbsp granulated sugar and stir for another 5 minutes until the onions caramelize.
Once the onions have caramelized, add ¼ cup of dry white wine and continue sauteeing until the wine cooks down.
Once the wine is gone from the pot, add 4 cups beef stock, 2 tsp worcestershire sauce, 1 ½ tsp chopped garlic, 2 bay leaves, 4 sprigs thyme, ¼ tsp salt (or more to taste), ¼ tsp pepper.
Turn the pot to HIGH and pressure cook for 15 minutes. Allow for a 10 minute Natural Pressure Release, then perform a Quick Release.
Spoon the soup into 5-6 oven-safe bowls. Place enough bread slices to cover the top of the soup, then sprinkle gruyere cheese on top.
Place the soup crocks on a cookie sheet and turn the oven to a High Broil. Place in the oven under the broiler and broil until the cheese starts to bubble and toast.