Preheat oven to 350° F.
Using a serrated bread knife, slice the bread lengthwise and crosswise, forming cubes, making sure NOT to slice all the way through the bottom. The bread "cubes" should still be attached at the bottom of the loaf.
Mix cheese, mushrooms, onions, poppy seeds and mustard in a large mixing bowl. Add the melted butter and mix until well combined. Stuff the bread with the cheese mix, being sure to stuff it all the way down to the bottom of each slice in the bread. Don't worry if a few of the bread cubes pop out while you are stuffing the cheese in, just push them back in. Once the cheese melts it will hold everything together.
Place the loaf on a baking sheet lined with parchment paper and bake for 30-35 minutes until cheese is melted and bread is golden brown.
Remove from oven and serve immediately.