Preheat oven to 350 degrees. Arrange your baguette slices on two large rimmed baking sheets; brush both sides with oil, and season with salt and pepper.
Bake until golden, 15 to 20 minutes, rotating sheets halfway through (if undersides are not browning, turn crostini over once during baking).
Spread a tablespoon of the horseradish dijon onto a toast.
Top with a slice of roast beef.
Top with a small dollop of more Maille Dijon Horseradish sauce, a few capers and some microgreens if you’d like (these are just for presentation).
Add a few crumbles of freshly grated parmesan cheese for another layer of flavor (optional)