Cut stale bread into 1" cubes. If not dried out, dry out overnight or place in a 300°F oven until dried out and crisp on the outside.
Melt 1/2 cup of butter, drizzle it on the bread, and toss to coat in a very large bowl.
Season the diced onion with a bit of salt and get it cooking in a large pan or skillet in the remaining tablespoon of butter over medium heat. Continue cooking over medium heat, add some of the stock if onions start to burn.
Once the onions are caramelized, transfer them to the bowl with the bread and give it a stir.
Deglaze the onion pan with a cup of stock, pour that stock into a bowl, and let it cool a bit.
Once the stock is no longer hot, beat in you eggs, the miso paste, and the remaining cup of stock.
Once you have a homogeneous mixture, pour it over the bread, toss everything together once more, and transfer it to a greased casserole dish.
Bake the whole thing at 375℉ until it’s set and the top is golden and crispy, about 40-45 minutes.