01 - Preheat the oven to 400°F. Line a large sheet pan with parchment paper or lightly grease it.
02 - Arrange green beans and red onion, if using, in a single layer on one side of the pan. Drizzle with 1 tablespoon olive oil, season with salt and pepper, then toss to coat evenly.
03 - Place salmon fillets skin-side down on the other side of the pan. Drizzle with remaining olive oil and season with salt and pepper.
04 - In a small bowl, whisk together melted butter, lemon juice, Dijon mustard, minced garlic, honey, parsley, 1/2 teaspoon salt, and 1/4 teaspoon black pepper until fully combined.
05 - Arrange lemon slices over the salmon and green beans. Brush or spoon half of the lemon butter sauce over the salmon and vegetables.
06 - Roast in the preheated oven for 15 to 20 minutes, until the salmon is opaque and flakes easily with a fork and the green beans are tender-crisp.
07 - Remove from oven. Drizzle the remaining lemon butter sauce over the salmon and green beans. Garnish with extra parsley and serve immediately.