In a large saucepan, heat oil over medium heat. Add onion and garlic and cook, stirring regularly, until onion is soft and transparent. Add paprika, cumin and cayenne and cook for 1 minute. Stir in sherry, vinegar and tomatoes. Bring to a simmer and cook until reduced by a third.
Remove pan from heat and allow to cool slightly. Process sauce with a stick blender until smooth. Transfer to a bowl, season with salt to taste and set aside to cool. When cold, add mayonnaise and stir to combine.
Season potatoes with salt then steam, fry or bake until tender.
Transfer to a bowl and fold in the desired amount of sauce (you may not need it all). Garnish with a sprinkling of paprika and herbs and serve.