Melt the butter in a saucepan over medium heat. Add the onions and cook and stir until tender, about 5 minutes. Mix the cornflour in a little cold water until dissolved, then add to the saucepan along with the tomatoes and stock, stirring until well mixed.
Mix in orange zest, juice, Tabasco® and oregano. Simmer, covered, for 10 minutes. Blend in a food processor, or using a handheld liquidiser, until smooth. Stir in the cream then season with salt and pepper to taste. Ladle into warmed bowls and serve.