1. Cut tuna in half lengthwise. Combine spices in a small bowl. Season tuna all over with spices and put in a 4- by 8-in. loaf pan. Chill, covered, 3 to 5 hours.
2. Preheat oven to 300°. Add bay leaf, herb sprigs, garlic, and zest to tuna, tucking them around fish; pour in enough oil to cover by 1/4 in.
3. Bake tuna 25 minutes, uncovered, lowering heat if necessary to keep oil from boiling. Let tuna cool to room temperature in oil.
4. Lift one piece of tuna from oil; separate into thick flakes. Top baguette slices with tuna, using more if needed, and arrange on a platter with olives and radishes. Scatter minced parsley over platter. Drizzle tuna with some of the oil, and flick some oil onto the olives and radishes too.
Make ahead: Through step 3, up to 10 days, chilled. Bring to room temperature before serving.
Note: Nutritional analysis is per serving.