Preheat oven to 400 degrees Fahrenheit.
In a medium-sized bowl, mix beer, mustard, syrup, pepper flakes, and salt until well incorporated.
In a large cast iron skillet over medium-high heat, cook bacon until crisp. Remove bacon to a plate and set aside.
In the same skillet, add Brussels sprouts, cut side down and sear for about 5 minutes, or until they are golden on the bottom. Meanwhile, crumble the bacon into the bear mixture. Pour the mixture over the sprouts and place the skillet in the oven until the liquid is reduced and thick, about 10–15 minutes. If it gets too dry, add a little water.
While the sprouts are in the oven, melt the butter in a medium skillet. Toast breadcrumbs in the skillet until golden.
Remove the Brussels sprouts to a serving platter and sprinkle breadcrumbs and parsley over the top of the sprouts. Serve immediately.