This makes a beautiful presentation and it's perfect for spring, although I've also made it with winter radishes as a Thanksgiving appetizer as well. You can use any type of radishes, but my favorite are the beautiful watermelon variety - a huge and gorgeous winter radish that I grow every single fall. In fact, last fall, they did so well that we still had a few left for our Easter feast! But you could always use spring radishes as well - Easter Egg varieties would be lovely, or French breakfast radishes if that's what you have.
It does take a few minutes to prepare, but you can save time by slicing the radishes and making the herb butter and "salad" topping ahead of time. Then, just toast and butter the bread before compiling and serving. (Remember to set the butter out a few hours ahead of time to allow it to soften for spreading)
It's from one of my all-time favorite cookbooks, and it's amazingly delicious for such a simple recipe, but the French know what they're doing! As the accompanying text with the recipe says, "Leave it to the French to come up with one of the most indulgent, rustic-chic snacks of all time." :-)