Add butter and stir until it is melted, then add flour and continue stirring until the roux (butter and flour paste) browns slightly and lets off a nutty aroma.
Add stock and tomatoes with juice and bring to a simmer stirring frequently so sauce does not stick to the bottom.
Add the herbs and spices.
Transfer to slow cooker and cook on high for four hours or low for eight hours.
Add prawns, okra, lemon juice and sea salt and fresh ground pepper to the slow cooker and continue for 20 minutes.