Using a box grater set in a bowl, carefully grate the cut sides of the tomatoes, stopping when you reach the skin. You should have about 2 cups of tomato pulp.
In a large, deep skillet, heat 1 tablespoon of the olive oil. Add the onion and half of the garlic and cook over moderate heat, stirring frequently, until they are softened and just beginning to brown, about 6 minutes. Add the tomato pulp and cook over high heat until it is thickened, about 5 minutes. Add the clam broth and boil until it is reduced by half, about 5 minutes.
Add the serrano, olives and halibut and simmer over moderate heat, stirring occasionally, until the fish is cooked through and the stew is thick, about 5 minutes longer.
Meanwhile, in a small bowl, using the back of a spoon, mash the remaining garlic to a paste with a pinch of salt. Whisk in the mayonnaise, pimentón and the remaining 2 tablespoons of olive oil.
Serve the fish stew in shallow bowls with a dollop of the pimentón mayonnaise.