Preheat your oven to 400°F (200°C).
Rinse the gold petite potatoes under cold water and cut them in half. Place them on a baking sheet.
Drizzle the potatoes with olive oil and season with ½ teaspoon of salt and ¼ teaspoon of black pepper. Toss to coat evenly.
Roast the potatoes in the preheated oven for about 20 minutes, or until tender and golden brown.
Melt 2 tablespoons of unsalted butter in a small bowl. Stir in the Dijon mustard, lemon juice, minced garlic, 1 teaspoon of salt, and ½ teaspoon of black pepper.
Place the salmon fillets on a plate and pour the Dijon mixture over them, ensuring they are well coated. Marinate for about 10 minutes.
After roasting the potatoes for 20 minutes, remove the baking sheet from the oven. Push the potatoes to one side and place the marinated salmon on the other side. Roast for an additional 12-15 minutes.
Once cooked, plate the salmon and potatoes, drizzling any remaining sauce over the top before serving.