Preheat the oven to 425°F. Select a heavy-duty, rimmed metal baking sheet (do not use parchment paper or a silicone mat, as direct metal contact is required for the Maillard reaction).

Drizzle the 2 tablespoons of extra virgin olive oil directly onto the bare baking sheet.

Add the Dijon mustard, grated Parmesan cheese, minced garlic, kosher salt, and coarse black pepper directly to the oiled pan.

Using a spoon or a spatula, physically slather and swirl these ingredients together across the center of the pan to create a coarse, unified paste.

Dot this paste evenly with the small cubes of unsalted butter.

Place the halved baby potatoes directly onto the smeared sheet pan sauce. Toss them aggressively until every potato is heavily coated in the mustard and butter mixture.

Arrange the potatoes entirely flat-side down, ensuring the cut surface makes direct, uninterrupted contact with the hot metal.

Roast in the preheated 425°F oven for 10 minutes. The direct heat will cause the butter and cheese to melt and bubble, establishing a deeply caramelized, crisp crust on the cut side of the potatoes.

Remove the pan from the oven. Using a thin metal spatula, carefully flip each potato over.

Return the sheet pan to the oven and roast for an additional 10 minutes, or until the potatoes are beautifully blistered on the outside and perfectly soft and pillowy on the inside.

Serve immediately, scraping up every drop of the reduced, crispy sheet pan sauce to drape over the finished dish.