How to Make It
Preheat oven to 425°F. Whisk together olive oil, vinegar, garlic, fennel seeds, and salt in a small bowl. Set aside 3 tablespoons of mixture; toss the remaining amount with Brussels sprouts, potatoes, and oregano sprigs in a large bowl. Spread evenly on a rimmed baking sheet and bake until just tender, about 15 minutes.
Meanwhile, brush salmon with reserved 3 tablespoons of the olive oil mixture. Remove baking sheet from oven; stir potatoes and Brussels sprouts and place salmon fillets in center; return to oven. Bake until salmon is cooked through, about 12 minutes. Serve with the vegetables.