Heat two tablespoons of the olive oil in a wok, add the shallot and garlic, and fry for 2-3 minutes, or until softened.
Add the chopped mushrooms and four of the sage leaves and fry for 4-5 minutes, stirring well. Season, to taste, with salt and freshly ground black pepper.
Add the fish stock and bring to a simmer. Continue to cook until most of the stock has evaporated. Stir in the chopped Serrano ham.
Meanwhile, heat the remaining olive oil in a frying pan and fry the remaining sage leaves and slices of ham for 1-2 minutes, or until just crisp. Remove the sage leaves and ham from the pan using a slotted spoon and set aside to drain on kitchen paper.
Add the sea bass fillets, skin-side down, to the same frying pan and fry for 1-2 minutes, or until crisp and golden-brown. Carefully turn the fillets over and fry for a further 1-2 minutes, or until the sea bass is cooked through.
To serve, spoon the mushroom mixture onto serving plates, sit a sea bass fillet on top of each pile of mushrooms. Top with the crispy sage and one slice of Serrano ham.