Cut 1½ tablespoons of the butter into 6 pieces and keep chilled in the refrigerator.
In a small bowl, whisk together the orange juice, mustard, lemon zest, lemon juice, and vinegar and set aside.
In a small saucepan, combine 1 cup of water, 1 tablespoon of the butter, and ½ teaspoon of salt and bring to a boil. Stir in the couscous, then cover the pan and remove from the heat. Let sit 5 minutes, then fluff with a fork. Cover to keep warm.
In a small (4- to 5-quart) Dutch oven or other deep, wide pot, heat the remaining 1½ tablespoons of butter and the olive oil over medium-low heat. When the butter has melted, add the carrots and ½ teaspoon of salt. Cover loosely and cook, uncovering frequently to gently stir, for 5 minutes. Add the shallots and a pinch of salt and stir, then cover loosely and continue cooking and stirring frequently until the carrots are tender and very nicely browned, about 8 to 10 minutes. Add the peas and the orange juice mixture and immediately stir well, quickly scraping up any browned bits. Remove the pan from the heat, add the cold butter, and stir until just melted and creamy. Stir in the tarragon.
Portion the couscous between two or three bowls or spoon onto one platter. (You may not use all the couscous if only serving two.) Spoon the carrot sauté over the couscous and garnish with hazelnuts (if using). Serve right away