In a medium saucepan, simmer onions, mushrooms and beer, uncovered, until the beer has been mostly absorbed.

While the veggies are simmering, preheat oven to 350°F.

Slice the loaf length-wise and width-wise, leaving a ½” to 1” uncut at the bottom of the loaf. (Do this carefully, so you don't slice too far through. What end up being your “corners” are the trickiest and most prone to tearing off.)

Place cut loaf on a foil-lined baking sheet.

Stuff cheese into the slits. Follow with the onion and mushroom mixture. Arrange any onions that don't fit on the top of the loaf. Sprinkle with cheese to top.

Wrap the loaf in aluminum foil, so that it is completely enclosed.

Bake at 350°F for 15 minutes.

Remove from oven and peel back the foil, exposing just the top of the loaf. Return to oven to bake 10 minutes more.

Let stand a few minutes to cool before serving.