Grill the bread and fennel for the panzanella first, then let them soak up the lemon-Dijon vinaigrette while you cook the salmon.
Heat grill to medium-high; place a large cast-iron skillet on grates to preheat. In a large bowl, whisk together lemon zest and juice, mustard, and sugar. Whisk in 1/4 cup oil; season with salt and pepper. Lightly brush fennel and bread with oil. Grill directly on grates, turning often, until charred but still crisp, 1 to 2 minutes for bread, 3 to 4 minutes for fennel. Let cool slightly, then thinly slice fennel lengthwise and chop bread into bite-size pieces. Toss with dressing, cucumbers, and dill. Season with salt and pepper.
Season fish on both sides with salt. Swirl remaining 1 tablespoon oil in skillet, then add fish, skin-side down; cover grill. Cook until skin crisps and fish is just cooked through, 4 to 5 minutes. Serve, skin-side up, alongside panzanella.