Preheat oven to 220°C (425°F). Wash potatoes and cut any larger ones in half so all pieces are uniform (about 2–3 cm). Toss potatoes with 2 tablespoons olive oil, 1 teaspoon salt, 1/2 teaspoon black pepper, and 1 tablespoon fresh thyme leaves. Spread in a single layer on a rimmed baking sheet. Roast for 25–30 minutes, turning once halfway through, until golden and tender. Keep warm.

Confit onion

If using store-bought confit d'oignon, warm gently in a small saucepan over low heat for 3–5 minutes to loosen. If making quick confit: thinly slice 2 medium onions, sweat in 1 tablespoon butter over medium-low heat with a pinch of salt until soft, then add 1 tablespoon brown sugar and 2 tablespoons balsamic vinegar and cook slowly 20–25 minutes until deep brown and jammy. Cool slightly before assembling.

Sandwich

Assemble sandwiches

Butter one side of each slice of marble rye. On the unbuttered side of four slices, evenly spread about 50 g duck rillettes per sandwich (200 g total divided). Top rillettes with a generous tablespoon (about 35–40 g) of warm confit onion. Layer 50 g Swiss cheese slices over the onion (enough to cover). Top with the remaining bread slices, buttered side up.

Grill

Heat a heavy skillet or griddle over medium heat until hot. Place sandwiches and cook 3–4 minutes per side, pressing gently with a spatula or a weighted press, until bread is golden-brown and cheese is fully melted. If the bread is browning before the cheese melts, lower heat slightly and cover the pan for 1–2 minutes to trap heat. Remove and let rest 1 minute, then cut each sandwich in half on the diagonal.

Salad and plating

Toss mixed baby greens with the vinaigrette (about 45 ml), season with 1 tsp salt and 1/2 tsp freshly ground black pepper to taste. Arrange a portion of greens on each plate alongside two sandwich halves and a generous serving of roasted potatoes.

Garnish potatoes and salad with chopped chives or parsley. Add sliced cornichons on the side if using.