In a large skillet, or two medium skillets, over very low heat, warm the olive oil and butter. Add the leeks and onions to the skillet(s) and cook, stirring occasionally, for about 1 hour, or until the onions are buttery and soft but not brown. Season with salt and pepper. Turn the heat to medium-high, add the wine and Cognac, and bring to a boil. Continue to boil for 3 minutes.