Melt butter and oil in a large saute pan and add onions. Cook covered over gentle heat for 40 minutes to one hour, until the onions are a deep golden brown. You may need to add water or stock to prevent the onions from burning. Stir in the fresh thyme, and toss with the pasta. Serve.
PEELING PEARL ONIONS: Add make an x in the bottom of your pearl onions, place in a bowl add 1/3 of a cup of water and cover and microwave for 12 minutes. You can peel them easily when slightly cooled.