Heat a large pot over low heat and add the butter. Once melted, stir in the onions, salt, pepper and thyme. Cook, stirring often, until the onions are softened and are starting to caramelize, about 20 minutes. Stir in the bourbon. Cook, stirring often again, for another 20 to 30 minutes until the onions are deeply caramelized and golden in color. Stir in the garlic.

Stir in the dry sherry and cook for about 5 minutes. Add in the beef stock and fresh thyme sprigs. Bring the mixture to a boil, then reduce it to a simmer and cook uncovered for 15 to 20 minutes, until the broth has reduced and thickened a bit.

While the soup is simmering, make your heart croutons! Preheat the oven to 350 degrees F. Use a heart-shaped cutter to cut out hearts in the bread, then place them on the baking sheet. Brush the bread with olive oil. Place in the oven for 10 to 12 minutes, until golden and crunch.

To serve the soup, ladle spoonfuls of the soup into oven-safe ramekins or crocks. Add a sprinkle of gruyere cheese on top, then a heart crouton. Add another sprinkle of gruyere on top. Repeat with remaining crocks.

Turn the broiler on your oven to high with a rack in the center of the oven.

Place the crocks on a baking sheet and stick them in the oven. Broil for 2 to 3 minutes, or until the cheese is golden and bubbly. Garnish with extra thyme if you wish. And serve!