Preheat oven to 400 degrees. Brush both sides of baguette slices with 2 tablespoons oil. Toast on a rimmed baking sheet until crisp, 7 to 8 minutes.
Stuff each pepper with a cheese square. Place 1 stuffed pepper on each toast. Bake until cheese melts, 8 to 10 minutes. Drizzle with remaining 2 tablespoons oil. Garnish with scallion.