Preheat oven to 350°. Spread walnuts in a single layer on a baking sheet and toast until aromatic and golden, about 5 minutes. Let cool.
Combine nuts, fennel fronds, spinach, cheese, olive oil, garlic and lemon juice in food processor and purée. (You might need to do this in a couple batches, depending on the size of the food processor bowl.) Add a touch more olive oil if needed to achieve a smooth consistency.
Stir in yogurt and season to taste with kosher salt and black pepper. Th in out with a bit of water if needed to achieve a good pouring consistency.
Boil potatoes in a pot of well-salted water until a knife inserted into the fattest potato meets no resistance. Pour into a colander and drain well. Place potatoes in serving dish and pour the fennel yogurt sauce over them. Garnish with kalamata olives.