Line a large rimmed baking sheet with parchment paper.
Combine mustard, garlic, lemon zest, lemon juice, rosemary, honey, and salt in a small bowl.
Combine rosemary, walnuts in another small bowl.
Place salmon on the prepared baking sheet, skin side down. Spread the mustard mixture over the fish and sprinkle with the walnut mixture, pressing to adhere.
Bake until the fish flakes easily with a fork, about 8 to 12 minutes, depending on thickness