Preheat oven to 250C. Season trout with sea salt and freshly ground black pepper. Place a slice of jamón in the stomach cavity of each trout and secure with 2 toothpicks, then place flour in a baking tray and carefully roll trout to coat, shaking off excess flour before transferring to a plate.
Heat olive oil over medium heat in an oven-proof frying pan large enough to fit both trout. Add pancetta and remaining jamón and sauté for 3 minutes, add garlic and sauté for another 3 minutes or until garlic begins to soften, then, using a slotted spoon, transfer pancetta, jamón and garlic to a bowl, reserving oil in frying pan.
Return frying pan to medium heat, add trout and fry for about 5 minutes on each side, or until the fish are golden and crisp on the outside and cooked through. Scatter trout with reserved pancetta, jamón and garlic, add cider and cook for another 2 minutes or until cider begins to bubble in pan. Transfer to oven and cook for 10 minutes, scatter with parsley and serve immediately.