Place eggs in a large saucepan; add water to cover by 1". Bring to a boil, cover, and remove from heat. Let sit for 10 minutes. Drain. Transfer eggs to a bowl of ice water and let cool completely, about 10 minutes; peel. Halve lengthwise and remove yolks. Coarsely chop 3 slices of bacon. Cook in a medium skillet over medium heat until browned and crisp. Transfer bacon to paper towels. Strain drippings through a fine-mesh sieve into a small bowl.
Finely mash reserved yolks, bacon fat, mayonnaise, Dijon mustard, and chopped scallions in a medium bowl; season with kosher salt and freshly ground black pepper.
Transfer to a large resealable freezer bag, then cut 1/2" off 1 corner. Pipe into whites; garnish with thinly sliced scallions and reserved bacon.
For perfectly crispy bacon, we like to bake it in the oven. See how to do it here!