Using a knife or kitchen scissors, remove the taproots and leafy tops from the radishes, forming perfectly round spheres. Wash and thoroughly dry the radishes.

In a large bowl, fold the softened butter with the radishes using a rubber spatula. Oil a mini loaf pan (about 5¾ by 3¼ by 2 inches) and line it with plastic wrap; press and pull the wrap as needed to eliminate air bubbles. Transfer the radish butter to the pan and spread it to the edges; smooth the top and tap the pan on the counter to remove any air bubbles. (Alternatively, transfer the butter directly to a sheet of plastic wrap or parchment paper and roll it into a log.) Refrigerate the terrine until very firm, 4 to 6 hours.

Using the plastic wrap, lift the terrine out of the pan and transfer to a work surface. Remove the wrap. Using a very sharp knife or gently serrated bread knife, cut the terrine into ¼-inch-thick slices. Serve with slices of fresh baguette and finish with fleur de sel.