Heat the oven: Preheat to 425°F (220°C). Line a large sheet pan with parchment for easy cleanup.
Make the lemon-garlic sauce: In a small bowl, whisk olive oil, minced garlic, lemon zest, lemon juice, Dijon, honey, salt, pepper, and smoked paprika.
Start the potatoes: Toss potatoes with about a third of the sauce.
Spread on the pan cut side down. Roast 12–15 minutes until they start to soften.
Add the veggies: Toss asparagus, bell pepper, and red onion with another third of the sauce. Push potatoes to one side, add veggies to the other.
Roast 8 minutes.
Season the salmon: Pat salmon dry. Brush with the remaining sauce. Add lemon slices on top if you like.
Finish roasting: Nestle salmon on the pan, skin side down if using.
Roast 10–12 minutes, until the salmon flakes easily and the veggies are tender-crisp. Thicker fillets may need an extra 2–3 minutes.
Rest and garnish: Let salmon rest 3 minutes. Sprinkle with fresh parsley or dill.
Taste and add a pinch more salt or a squeeze of lemon if needed.
Serve or portion: Serve immediately, or divide into meal prep containers with equal portions of salmon and veggies.