Grill the sausage and hold warm until ready to serve
Place the quinoa in a strainer and rinse several times with cold water. Place in a medium saucepan with 1 1/2 cups water and a pinch of salt. Bring to a boil, cover and simmer 15 minutes, until the grains display a threadlike spiral and the water is absorbed. Remove from the heat, remove the lid and place a dish towel over the pan, then return the lid to the pan and let sit for 10 minutes or longer undisturbed. Transfer to a salad bowl and fluff with forks.
Steam the asparagus for 3 to 4 minutes, until just tender. Rinse with cold water, pat dry and cut on the diagonal into 1-inch pieces. Add to the quinoa, along with the pine nuts, scallions and tarragon. Whisk together the lemon juice, lemon zest, garlic, salt, olive oil, buttermilk and pepper. Shortly before serving, toss with the quinoa and asparagus mixture.
Cut the grilled sausage, mix into the quinoa and serve.