Using a knife, remove the roots and leafy tops from the radishes then wash and thoroughly dry the radishes.
In a large bowl, fold the softened butter with the radishes and chopped chives using a rubber spatula.
Line a mini loaf pan (or any small glass dish or silicone mold you love) with plastic wrap, leaving overhang on the sides for easy lifting.
Transfer the radish butter to the dish and spread it to the edges; smooth the top and tap the pan on the counter and gently press down to remove any air bubbles.
Cover and refrigerate for at least 4 hours (or overnight) until very firm.
Using the plastic wrap, lift the terrine out of the pan and transfer to a cutting board. Gently remove the wrap.
Using a very sharp chef's knife, cut the terrine into ¼-inch-thick slices.
Finish with a generous pinch of fleur de sel.
Serve on toasted bread like sourdough or baguette slices.