Wash onions, leaving outer skins on. Cut about 1/4 of the top off the blossom end. Place onions. and tops cut side down on baking sheet. Drizzle with 1 tablespoon of olive oil. Bake in a 350 degree oven for 45 minutes to 1 hour until soft inside. Remove from heat.
Cook capellini pasta until al dente, about 4-5 minutes. Rinse in cool running water. Drain well, and toss with 1 tablespoon of olive oil. Set aside. Cook bacon or pancetta in saute' pan until crisp. Drain and set aside.
When onions have cooled, scoop out the centers leaving 2 layers of onion and the skins intact. Reserve shells. Julienne insides of cooked onion. Place in a heavy saucepan with butter, salt, pepper and thyme. Cover and cook over low heat, stirring often until caramelized.
When ready to serve, re-heat onion shells and tops in a 350 degree oven for about 10 minutes. Heat caramelized onions with balsamic vinegar, chicken stock and pancetta. Test for seasonings. Add pasta and heat through in sauce. Add parsley, toss well. Place pasta inside onion shells. Serve with tops slightly askew.