How to Make It
Preheat oven to 400°F. Arrange bacon in a single layer on a foil-lined rimmed baking sheet. Bake until crisp, 15 to 20 minutes, depending on thickness. Drain on a paper-towel-lined plate.
Meanwhile, melt butter in a medium pot over medium. Whisk in flour and cook, whisking constantly, until slightly darkened, 1 to 2 minutes. Gradually whisk in milk and bring to a simmer. Cook, whisking often, until mixture is smooth and slightly thickened, 1 to 2 minutes. Remove from heat and whisk in mustard, salt, pepper, and half the cheese.
Arrange 4 slices bread on a rimmed baking sheet. Spread with half the cheese sauce. Top with bacon and remaining bread. Spread remaining cheese sauce over sandwiches. Top with a few onion rings and dust with remaining cheese.
Bake until bubbly, 10 to 12 minutes. Increase oven to broil. Broil until golden brown, 1 to 2 minutes. Let cool slightly before serving.