Place a rack in the upper third of your oven and preheat the oven to 400 degrees F. Brush a rimmed baking sheet with 1/2 tablespoon olive oil. Arrange the baguette slices in a single layer on the sheet, then brush the baguette tops with the remaining 1/2 tablespoon olive oil. Bake for 5 minutes, then remove from the oven, flip the crostini over, and continue baking until lightly golden, about 5 to 6 additional minutes. Let cool.
In the bowl of a food processor fitted with a steel blade, whip the feta for 30 seconds. Add 1/2 tablespoon olive oil, milk or cream, and black pepper.
Whip again until creamy and fully combined, about 1 minute, scraping down the bowl as needed.
To assemble the crostini: spread each baguette slice with whipped feta. Add any toppings and season with a pinch of additional salt or pepper to taste. Enjoy
TO MAKE AHEAD: Prepare the whipped feta mixture as directed up to 1 day in advance. Store in the refrigerator. When you're ready to serve, prepare the crostini and top with the feta mixture.
TO STORE: Store leftover crostini at room temperature in an airtight storage container for up to 2 days. Refrigerate any leftover whipped feta mixture in an airtight storage container for up to 2 days.
TO FREEZE: You can freeze crostini. Freeze leftover crostini in an airtight, freezer-safe storage container for up to 3 months. Thaw and enjoy as desired.