1. Bring 2 inches of water to a boil with fennel stalks in a large saucepan. Add chicken and 3 tablespoons salt (if chicken is not fully submerged, add water to cover). Reduce heat to medium-low; simmer until a thermometer inserted into thickest parts of chicken registers 165 degrees, 20 to 25 minutes. Transfer chicken to a cutting board; let cool 10 minutes.

2. Whisk together yogurt, oil, and tarragon; season with salt and pepper. Remove chicken skin, then remove meat from bones and thinly slice against the grain. Divide lettuce, chicken, sliced fennel, and plums among 4 plates. Top with dressing, almonds, and fennel fronds; season with salt and pepper and serve.