How to Make It
Prepare the Salmon: Fold a 30- x 18-inch piece of parchment paper in half lengthwise; fold in half again crosswise (short end to short end) to create a 4 layer thick piece. Place folded parchment in bottom of a 6-quart slow cooker, letting ends extend partially up sides.
Place half of lemongrass, fennel, and scallions in an even layer on parchment in slow cooker. Add water and wine. Sprinkle salmon fillets with salt and pepper; place on lemongrass mixture. Top salmon with remaining lemongrass, scallions, and fennel. Cover and cook on HIGH until salmon flakes easily with a fork, 1 to 2 hours. Using parchment paper liner as handles, lift salmon from slow cooker, allowing liquid to drain off. Discard mixture in slow cooker. Set salmon aside.
Prepare the Bagel Sandwich: Combine the softened cream cheese, chives, radishes, dill, lemon zest, and lemon juice in a small bowl. Spread mixture on bottom halves of 4 split everything bagels. Divide arugula, 8 ounces flaked reserved salmon, red onion, and English cucumbers among bagel bottoms; cover with tops.