Cook tortellini according to package directions to al dente. Drain and allow to cool, but do not run under water. (Mix in a little olive oil to prevent sticking and drying.)
Meanwhile, marinate the sundried tomatoes and garlic in extra virgin olive oil. When tortellini have cooled enough to handle, add the arugula, onion, prosciutto, asiago cheese and the sundried tomato/garlic/olive oil mixture, mixing well and adding salt and freshly ground pepper to taste.
Allow to sit, covered and chilled for about an hour so flavors can blend.