Use a 6-quart slow cooker. Spread 6 sheets of foil or parchment paper onto your kitchen countertop and place a piece of fish in the center of each. In a small mixing bowl, combine 2 teaspoons lemon zest and 1 tablespoon lemon juice. Stir in the Dijon mustard. Spoon this sauce on top of each piece of fish, and top with a tiny handful of cheese. Fold the foil or parchment paper over and crimp the sides to make a fully enclosed packet.
Stack the packets into the insert. Do not add any water. Cover and cook on high for 2 hours or until the fish flakes easily with a fork. Serve with the remaining lemon juice squeezed on top.