1. Trim stems and root ends off radishes; rinse well, and cut in half crosswise. To make each half sit flat, trim off rounded end. If crème fraîche or sour cream is runny, use a small melon baller to scoop out centers of radishes, making depressions to hold filling; if crème fraîche is thick, leave radish halves flat.
2. In a bowl, mix crème fraîche and lemon peel and juice. Add salt to taste.
3. Top or fill radish cups equally with crème fraîche mixture (about 1/2 teaspoon in each), then top equally with tobiko (a scant 1/4 teaspoon on each). Arrange on a platter.