Season the shanks with salt and pepper then lightly dust with flour.
Melt the butter in the pressure cooker and brown the meat over medium/high heat. Set aside.
Add the onion and garlic to the cooker and cook 5 minutes or until onion is soft. Reduce heat to medium. Return the meat to the cooker and add water, wine, tomato paste and bouquet garni. Close the cooker and cook on low pressure 20 minutes. Remove the bouquet garni before serving.