Place small yellow potatoes in a large pot and cover them with cold, salted water. Bring to a boil over high heat, then reduce to medium and simmer for 15-20 minutes, or until fork-tender but not falling apart. Drain the potatoes and allow them to cool slightly.

Preheat your oven to 425°F (220°C) while arranging the drained potatoes on a baking sheet lined with parchment paper. Using a flat-bottomed object, gently smash each potato until its edges are crisped.

In a large skillet, melt unsalted butter over medium-low heat, then add sliced yellow onions. Cook slowly, stirring occasionally, for 20-30 minutes until the onions are golden brown. Add fresh thyme and a splash of dry white wine, allowing it to reduce.

Once the onions are caramelized, arrange the smashed potatoes evenly on your baking sheet. Top each potato with the caramelized onions and shredded Gruyère cheese. Bake for 15-20 minutes, or until the cheese is bubbly and golden.

Remove from the oven and garnish with fresh thyme leaves and additional black pepper. Serve warm.