Heat oven to 400° F. In a large roasting pan, toss the fennel, lemons, garlic, 1 tablespoon of the oil, and 1/4 teaspoon each salt and pepper. Roast until the fennel begins to soften, about 8 minutes.
Season the salmon with 1/2 teaspoon salt and 1/4 teaspoon pepper and nestle in the fennel. Roast until the salmon is opaque throughout, 12 to 15 minutes.
Squeeze the garlic out of the skins into a small bowl and mash to a paste. Squeeze the lemon pulp and juice into the bowl. Stir in the honey, rosemary, the remaining 2 tablespoons of oil, and 1/4 teaspoon salt. Place the salmon and fennel on the greens. Drizzle with the dressing.
Make Ahead: This dish is also delicious served at room temperature. Roast the fish, fennel, and lemon and make the dressing in advance. Dress the greens and the fish just before serving.