Bring a large pot of salted water to a boil. Have ready a bowl of well salted ice water and a slotted spoon next to the stove. Drop peas into boiling water and cook until crisp-tender, about 1 minute. Use the spoon to transfer peas to ice water to cool completely. Drain well and pat dry with a clean kitchen towel.
When cool enough to handle, put the peas on a cutting board and slice them in half, crosswise.
In a medium bowl, whisk together garlic and lemon juice. Stir in salt and pepper. Whisk in 3 tablespoons olive oil. Add the halved peas, fennel, dill and scallions, and toss well. Taste and add more lemon juice, olive oil or salt if needed.
In a small bowl, stir together the yogurt, 1/4 cup feta, lemon zest, and the remaining 1 tablespoon oil. Season with salt, pepper and lemon juice, to taste.
Spread the yogurt on a serving platter, and spoon the peas on top. Scatter remaining 1/4 cup feta on top, drizzle with more oil, and top with herbs. Serve as a side dish or scooped onto crusty bread as an appetizer.