Preheat oven to 425°F and butter twelve 1/3-cup muffin tins.
In a skillet cook bacon over moderately low heat until crisp and transfer to paper towels to drain, reserving 1/4 cup bacon fat. Crumble bacon.
In a bowl whisk together flour, baking powder, sugar, salt, Gruyère, scallion, and bacon. In a small bowl whisk together milk, egg, mustard, and reserved bacon fat. Add milk mixture to flour mixture, stirring until just combined (do not overmix).
Divide batter evenly among muffin tins and bake in middle of oven 15 to 20 minutes, or until golden and a tester comes out clean.