Line a rimmed 17 x 12-inch baking sheet with parchment paper. Place salmon in the center of the pan.
In a mixing bowl, whisk together 1 1/2 tablespoons olive oil, lemon zest, lemon juice, 2 cloves minced garlic, Dijon mustard, onion powder, and a 1/2 teaspoon each salt and pepper. Brush mixture evenly over the top of the salmon. Top with lemon slices.
In an oblong dish, toss together asparagus with remaining 1 tablespoon olive oil, remaining 2 cloves minced garlic; season with salt to taste then place around the salmon.
Bake in preheated oven for 10 minutes. Remove from oven, toss asparagus then sprinkle asparagus with parmesan. Return to the oven and bake until salmon has cooked through, about 5 to 10 minutes longer. Cut salmon into portions. Serve immediately with asparagus.