Cheesy French Onion Pull-Apart Bread

Caramelize the onions
-1 Melt the butter in a large nonstick skillet over medium heat.
-1 Add the thinly sliced onions, seasoning with the salt. This will help them cook down more quickly.
-1 Stir the onions occasionally until they have begun to caramelize, about 15-20 minutes.
-1 When the onions are a deep brown, add the red wine to deglaze the pan. (If you don't want to use wine, use the same amount of beef stock.)
-1 Cook until all the wine has evaporated out of the onions, then remove from the heat to let cool.
Prepare the bread
-1 While the onions cook down, slice the sourdough into 1″ squares. Keep in mind that we do not want to cut all the way through the bread.
-1 Use a serrated knife to slice 1" slices on the diagonal into the bread, careful not to cut all the way through the bottom.
-1 Spin the bread 90 degrees, and cut slices going this direction, forming the squares or diamonds.
Make the cheese sauce
-1 Preheat the oven to 350°F. Place two pieces of aluminum foil on a baking sheet, shaping them into a plus sign. (These will wrap around the bread as it bakes.)
-1 In the same pan that the onions were cooked in, add the shredded cheeses and milk.
-1 Stir constantly, allowing the cheese a chance to melt.
-1 Season with the thyme and black pepper. Remove from the heat.
-1 Once the cheese sauce has finished cooking, pry open the bread pieces using your fingers or two butter knives and drizzle the cheese in. Top with the onions.
-1 Complete this process until every nook and cranny of the bread has been filled with cheese and onion.
-1 Wrap the bread in the prepared foil, and bake for 15-20 minutes.
-1 Enjoy immediately!