Peel and cut the avocados in half and remove the pits. Cut the flesh into cubes and put them in the container of a food processor. Add the lemon juice and ½cup stock. Process until fine in texture; pour mixture into a saucepan.
Melt the butter in another saucepan and add the onion. Cook until wilted and sprinkle with flour. Stir to coat the pieces and add the remaining 3cups stock, stirring rapidly with a wire whisk. When blended and smooth, remove from the heat.
Add the sauce to the avocado mixture. Add the sour cream, tomatoes, salt, and pepper. Add Tabasco sauce to taste. Serve hot or chilled.