In a saucepan, over medium heat, add the oil. When the oil is hot, add onions. Season with salt and pepper. Saute for about 10 to 12 minutes, or until caramelized. Add the stock and bring to a boil. Reduce the heat to medium low and simmer for 10 minutes. Remove from the heat and using a hand-held blender, puree until smooth. Strain through a sieve and cool completely. Taste, and reseason if necessary.