Evenly space the bacon on a baking sheet lined with a wire rack and bake until crispy, 15 to 18 minutes. Drain the bacon on paper towels, reserving the bacon fat from the pan. Reduce the oven temperature to 350 degrees F. Once cooled, finely mince the bacon.
For the caper spread: In a bowl, combine the cream cheese, capers, chives and honey mustard, and season with salt and pepper. With a rubber spatula, mix all the ingredients until combined. Adjust the seasoning if necessary.
For the sandwich build: Cut the crusts from the bread. Brush slices on one side with the reserved bacon fat and transfer to the oven to warm through, about 5 minutes. Smear the fat side of four slices with some caper spread. Layer with 1 1/2 ounces of smoked salmon, some red onion, tomato, minced bacon and arugula. Top with another slice of bread and cut each sandwich into thirds, to make finger sandwiches.