Irish Stout Onion Soup

3 TBSP butter, divided	1 1/2 TBSP stone ground mustard
4 large sweet yellow onions, sliced	2 bay leaves
3 TBSP brown sugar	salt and pepper to taste
3 1/2 cups beef broth, divided	For the topping
3 cloves garlic, minced	4 slices crusty bread, cut into cubes
1 TBSP fresh thyme leaves	1 TBSP butter
1/4 cup all-purpose flour	leaves from 2 sprigs of thyme
20 ounces Irish stout	3/4 cup grated white Irish cheddar
2 TBSP apple cider vinegar

Directions:

Place 2 TBSP of butter in a large Dutch oven or heavy bottomed pot over medium-low heat.
Once melted, add onions and brown sugar. Stir to combine and cook until deep golden brown and caramelized, about 45 minutes – 1 hour. Stir occasionally while cooking and add beef broth as needed to keep from burning.
When onions are done, add garlic, thyme and remaining 1 TBSP of butter to the pot. Cook for 1-2 minutes until fragrant.
Add flour, stir to coat all the onions and cook 2 more minutes.
Add the stout, remaining 3 cups of beef broth, apple cider vinegar, mustard, bay leaves, salt and pepper. Stir and bring to a boil. Once boiling, lower to a simmer and cook/reduce for 30 minutes.
Season to taste with salt and pepper and turn off heat.
Preheat oven to high-broil.
Make the topping by placing the butter and thyme in a medium skillet over medium heat.
Once melted, add the bread cubes and toast in skillet until golden brown on all sides.
Ladle soup into oven safe bowls/crocks, top with the toasted croutons and divide the cheese evenly over the croutons in each bowl.
Place bowls on a baking sheet and broil for 3-5 minutes until cheese is melted and bubbly.