Cook onions slowly, stirring every 5–7 minutes, until they are soft and deeply golden, about 30–40 minutes. If they start to stick, reduce heat and add a splash of water.

When onions are soft and slightly caramelized, stir in brown sugar, red wine vinegar, thyme, salt and pepper. Cook another 3–5 minutes until jammy. Remove from heat and let cool slightly.

Pommes de terre (potatoes)

Preheat oven to 220°C (425°F). Toss halved baby potatoes with olive oil, minced garlic, chopped rosemary or thyme, salt and pepper on a rimmed baking sheet in a single layer.

Roast until potatoes are golden and tender when pierced with a fork, about 25–30 minutes, turning once halfway through. Keep warm.

Sandwich assembly & grilling

Lay out 4 slices of marbled rye bread. Spread about 50 g (approximately 2 tbs) of rillettes de canard evenly on each slice.

Top the rillettes with 50 g (about 1/4 of the cheese) of sliced Swiss cheese per sandwich (adjust so cheese covers the surface). Spoon 2–3 tbs of the confit d'oignons over the cheese layer.

Top with the remaining bread slices to form 4 sandwiches. Butter the outside faces of each sandwich evenly with softened butter.

Heat a large skillet or griddle over medium heat. Place sandwiches butter-side down and press gently with a spatula or a heavy pan. Cook until the bottom is golden brown, about 3–4 minutes.

Flip sandwiches and cook the other side until golden and cheese is melted, another 3–4 minutes. If the cheese needs more melting without over-browning, lower heat and cover pan for 1–2 minutes.

Transfer to a cutting board and let rest 1 minute. Cut each sandwich in half on the diagonal.

Verdurette & plating

Toss mixed baby greens with lemon juice, extra-virgin olive oil, salt and pepper to taste.

Arrange each plate with one grilled sandwich half (or two halves per plate), a generous handful of dressed greens, and a portion of roasted potatoes (about 150 g per serving). Spoon any remaining confit d'oignons alongside or on the plate as garnish.