Our favorite brand of pearl couscous is Roland Israeli Couscous. For efficiency, let the shallots pickle while you prepare the remaining ingredients.

Bring vinegar, sugar, and pinch salt to simmer in 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 shallots cool, heat couscous and 1 tablespoon oil in medium saucepan over medium heat, stirring frequently, until about half of grains are golden brown, 5 to 6 minutes. Add water and ½ teaspoon salt; stir to combine. Increase heat to high and bring to boil. Reduce heat to medium-low, cover, and simmer, stirring occasionally, until water is absorbed, 9 to 12 minutes. Remove saucepan from heat and let stand, covered, for 3 minutes. Transfer couscous to rimmed baking sheet and let cool completely, about 15 minutes.

Whisk remaining 3 tablespoons oil, lemon juice, mustard, pepper flakes, and remaining ⅛ teaspoon salt together in large bowl. Add cooled couscous, arugula, mint, peas, 6 tablespoons pistachios, ½ cup feta, and shallots and toss to combine. Season with salt and pepper to taste and transfer to serving bowl. Let stand for 5 minutes. Sprinkle with remaining ¼ cup feta and remaining 2 tablespoons pistachios and serve.