A mandoline is ideal for slicing radishes into perfect thin rounds, but a sharp knife also works. The vegetables can be trimmed and sliced a few hours ahead. To keep them crisp, wait to toss them with the other ingredients until just before serving.
Steaming fish on a bed of basil gives it a nice flavor without adding calories. If you have a large bamboo steamer or a multilayer steamer, you can multiply the recipe. (The vinaigrette makes enough for four to six servings.) Halibut also works great with this technique; just steam the fish for three minutes before adding the peas.