For the pickled shallots:
Bring vinegar, sugar and a pinch of salt to a simmer in a small saucepan over medium-high heat, stirring occasionally until sugar dissolves. Remove pan from heat, add shallots and stir to combine. Cover and let cool completely, about 30 minutes. Drain and discard liquid. While it cools make the couscous.
For the couscous:
Heat couscous and oil in medium saucepan over medium heat, stirring frequently, until about half of grains are golden brown, 5-6 minutes. Add water and salt, stir to combine. Increase heat to high, and bring to a boil. Reduce heat to medium-low, cover and simmer, stirring occasionally, until water is absorbed, 9-12 minutes. Remove saucepan from heat and let stand, covered, for 3 minutes.
For vinaigrette:
Whisk or shake together the oil, lemon juice, mustard, pepper flakes and a pinch of salt.
To assemble:
Combine cooled couscous, arugula, mint, peas, half the pistachios, half the feta and shallots and toss. Season with salt and pepper to taste and transfer to serving bowl. Let stand for 5 minutes. Sprinkle with remaining feta and pistachios and serve.
