Add all ingredients except buttermilk to a medium bowl and whisk until combined. Add 1/2 cup buttermilk and whisk thoroughly. Test the consistency and add more buttermilk until it reaches your preferred pourability. I used 3/4 cup, which makes it pourable but tangy rich. Taste, and add more salt if desired. Let sit in the fridge for at least 2 hours before serving. (For best results, taste again after the rest, and add more salt a pinch at a time as needed.)

Cook the tortellini according to package directions, drain, and set aside to cool. Add the vegetables to a medium mixing bowl, season with a pinch of salt and pepper, and toss. Fold in the pasta, taking care not jar the tortellinis into unfolding. Pour some of the dressing and fold it into the pasta salad until lightly coated. Serve immediately at room temperature with the extra dressing on the side, or refrigerate the salad and remaining dressing for at least an hour. Use the extra dressing to loosen the salad, and serve.