Heat oven to 350 degrees. Line a large baking sheet with parchment paper.
Cook the bacon until the fat is rendered out, but not until it is browned and crispy. It will crisp up when it bakes; set aside.
Slice the bread evenly in half lengthwise along the sides and place both cut side up on the prepared baking sheet. Slice each into crosswise pieces at about 1-inch intervals, cutting almost to the bottom crust.
Melt butter in a frying pan over medium heat. Add onion and saute until softened. Stir in the mustard seed and poppy seed and cook, stirring constantly for another minute. Remove from heat and stir in mustard. Spread mixture between cut slices of bread. Insert the cheese slices (cut as needed to fit) between slices. Scatter bacon evenly over tops of loaves. Bake at 350 degrees until cheese melts and bacon is crisp, about 10-12 minutes. Serve hot.
Recipe by The Culinary Cellar at https://theculinarycellar.com/bacon-and-cheese-pull-apart-bread/