Pommes de Terre Fondantes (Fondant Potatoes)

Adjust oven rack to middle position and preheat oven to 450°F (230°C). Place stock in a liquid measuring cup or small bowl and sprinkle gelatin over top. Set aside.

Using a sharp knife, cut off ends of potatoes to give them flat sides, then halve potatoes crosswise. Pat dry with paper towels.

In a 12-inch stainless steel straight-sided sauté pan or skillet, or cast iron skillet, heat oil or fat over medium-high heat until just beginning to smoke. Season potatoes with salt and pepper and add to skillet, broad side down, in a single layer with space between each piece. Cook, without moving, until potatoes begin to brown around edges, 4 to 5 minutes. Continue to cook, rotating and swirling pan gently to promote even browning and prevent sticking, until potatoes are deeply browned on bottom side, 3 to 4 minutes longer, adjusting heat as needed if some of the pieces brown too quickly.

Using a thin metal spatula, flip potatoes onto second flat side. Add butter and cook, swirling constantly, until butter is melted and begins to foam, about 1 minute. Add garlic and thyme, and cook until fragrant, 30 seconds to 1 minute. Add stock and bring to a boil.

Transfer skillet to oven and roast until potatoes are completely tender, offering little to no resistance when poked with a paring knife, and liquid is reduced to a saucy consistency, 25 to 30 minutes.

Return skillet to stovetop. Using a thin metal spatula or tongs, transfer potatoes to a serving platter, broad side up, leaving sauce in the skillet; discard garlic. Bring sauce to a simmer over medium heat and cook, swirling and stirring constantly, until sauce is emulsified, 30 seconds to 1 minute. If emulsion appears broken or sauce is too thick, add more stock or water, 1 tablespoon (15ml) at a time, until you reach the desired consistency. Season to taste with salt and pepper and pour sauce over and around potatoes. Sprinkle with flaky sea salt (if using, otherwise season with a little more kosher salt), and serve.