1. Put honey into a small pot and cook over medium heat until slightly thinned and just hot, about 3 minutes. Add rosemary and lemon zest and stir to combine. Remove from heat and let infuse for 15 minutes. Pour honey through a fine sieve into a small bowl and let cool completely. Discard solids.
2. Arrange salmon filets, flat side down, in a shallow dish in a single layer. Season to taste with salt and pepper and drizzle with 2 tbsp. of the infused honey (reserve remaining honey for another use). Cover dish with plastic wrap and refrigerate for 1 hour.
3. Preheat grill to medium-low and rub grates generously with olive oil. Place salmon on grill, rounded side down, and cook, rotating to create crosshatched markings and turning once, until dark golden brown and medium rare, 2 to 3 minutes per side. Divide salmon between 4 plates and drizzle with lemon juice. Serve immediately or at room temperature.
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