3 tbsp unsalted butter
3 spanish onions, sliced thin
1 tsp sugar
1 tsp salt
3 cloves garlic, minced
2 tsp fresh thyme, finely chopped
1/4 cup dry sherry
1/2 cup dry white wine
1 loaf italian or french bread (1 lb), cut into 1 inch cubes
6 large eggs
3 1/2 cup half and half or whole milk
1 1/2 tbsp dijon mustard
1 tbsp worcestershire sauce
1 tsp fish sauce
1/2 tsp kosher salt
1 tsp black pepper
1 1/2 cups grated gruyere
In a large skillet, melt butter, then add all the onions, sugar, and 1 tsp salt. Cover and cook, stirring every couple minutes or so until softened. Remove lid and cook, stirring frequently until onions are browned. Lower heat or add water if onions look like they are starting to burn. Add garlic and thyme and cook 1 minute more. Add sherry and wine and cook until almost completely evaporated, about 3 minutes. Remove from heat.
Preheat oven to 350F. Spread bread cubes on the baking sheet and bake until dryed out but not browned, about 8 minutes.
Whisk together the eggs, milk, mustard, worcestershire sauce, fish sauce, 1/2 tsp salt, and pepper. Add the bread cubes and soak from 15-30 minutes.
Butter a 9×13 inch baking dish and add half the bread. Cover bread with half the caramelized onions and half the cheese. Layer bread on top of that, and add the rest of the onions and cheese. Drizzle any remaining egg mixture on top, then bake until bread is crispy and eggs are set, about 35 minutes. Serve hot.