Bring a large pot of salted water to a boil for the penne. Before you boil the pasta, set the shrimp in a strainer and dip them in boiling water to cook them through, a few minutes. Drain, cool, pat dry and cut into 1/2-inch pieces. Add the penne to the same boiling water and cook for about 8 to 10 minutes or until tender but still firm. In a food processor puree the green onions, celery, oil, lemon juice, and thyme. Season to taste with Tabasco sauce.
Drain pasta and rinse under cold water to cool thoroughly. Drain again and pat dry.
Combine the pasta, shrimp and toss with the dressing. Right before serving, peel, pit and dice the avocados and add them to the pasta. Toss gently and adjust the seasoning.