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.
Tip: 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.