Spread the pecan pieces onto a baking sheet and place in a cold oven.
Heat the oven to 350 degrees F (175 degrees C) to begin roasting the pecans. Roast until aromatic, 20 to 30 minutes.
Melt 1 tablespoon butter in a large skillet over medium heat. Cook the mushrooms, onion, and garlic in the melted butter until softened, 5 to 7 minutes. Transfer to a bowl and set aside.
Return the skillet to the heat. Melt 2 teaspoons butter in the skillet. Stir the water into the butter; bring to a boil. Put the couscous in a glass bowl; pour the butter and water mixture over the couscous. Immediately cover the bowl with plastic wrap and allow to sit until the couscous absorbs all of the moisture, 7 to 10 minutes. Fluff with a fork. Stir the toasted pecans, the mushroom mixture, the pesto, and Parmesan cheese through the couscous. Season with salt and pepper to serve.