Combine the yogurt, ⅓ cup of the peas, 3 tablespoons of the oil, and the shallot in a food processor. Puree until a smooth, pale green sauce forms. Spoon the sauce into a large bowl. Combine the remaining 3 tablespoons oil, pine nuts, pepper flakes, and 3 pinches of salt in a heavy small skillet over medium-low heat. Cook until the nuts are golden brown and the oil is deep red, about 6 minutes. Remove from the heat.

Steam the potatoes over boiling water until almost cooked through (pull one out, cut in half and taste), about 12 minutes. Add the remaining peas, cover and steam until the potatoes and peas are just cooked though, about 3 minutes longer. Set the steamer basket in a colander and let cool for no more than 5 minutes.

Add about half of the potato and pea mixture to the yogurt sauce and stir to coat. Mix in the remaining potato mixture. Add the basil and feta and mix gently. Season to taste with salt and pepper. Spoon the potato mixture into a large bowl. Spoon the pine nuts and oil over and serve warm or at room temperature.