Combine the Brussels sprouts and 2 tablespoons water in a microwave-safe bowl; cover with plastic wrap and put the bowl on a plate. Prick the potatoes with a fork and place around the bowl on the plate. Microwave the Brussels sprouts and potatoes until the Brussels sprouts are bright green, about 7 minutes, then continue cooking the potatoes until just tender, 3 to 4 more minutes. Let both cool slightly, then drain the Brussels sprouts and chop the potatoes.
Heat 2 tablespoons olive oil in a large nonstick skillet over medium-high heat. Add the Brussels sprouts and potatoes; season with salt and pepper and cook, stirring, until browned, about 5 minutes. Add 1 more tablespoon olive oil, the apple, serrano and thyme, and cook until the apple softens, 5 minutes.
Meanwhile, heat the remaining 1 tablespoon olive oil in a separate large nonstick skillet over medium-high heat. Season the fish with salt and pepper; add to the skillet and cook, turning once, until just cooked through, about 8 minutes.
Divide the hash among plates; top with the fish. Add the cider and 2 teaspoons vinegar to the fish skillet; simmer until reduced, about 2 minutes. Remove from the heat and swirl in the butter and the remaining 1/2 teaspoon vinegar; season with salt. Spoon over the fish and hash; top with parsley.