Melt butter in medium saucepan over medium heat, and then add onions. Cook, stirring occasionally, until softened, about 8 minutes.Sprinkle flour over the onions and stir until smooth. Cook, stirring constantly, until flour is golden brown, at least 2 minutes. Adjust heat so mixture does not burn.Gradually whisk in 4 cups of the stock. Cook, whisking, until mixture is smooth and thick, about 5 minutes. If it is too thick, add more stock.Stir in bourbon, cream, and thyme. Simmer 2 minutes, stirring gently. Season with a little salt and plenty of pepper. Serve warm.This gravy can be made up to three days ahead. Cool, pour into a glass jar with a tight-fitting lid, and refrigerate. Pour into a saucepan and heat gently over low heat. Do not microwave the gravy in the jar. Check the seasoning before serving and sprinkle with a little more thyme for looks.