Pommes de Terre Fondantes (Fondant Potatoes)

Name: Pommes de Terre Fondantes (Fondant Potatoes)
Prep time: 5 min
Cook time: 40 min
Total time: 45 min
Cuisine: French
Ingredients: 1 1/2 cups homemade chicken or beef stock, or store-bought low-sodium chicken broth, plus extra as needed; 1/4 ounce (2 1/2 teaspoons; 1 packet; 7g) unflavored gelatin, such as Knox; 2 1/2 pounds (1.1kg) Yukon Gold potatoes, about 2 1/2 to 3 inches long each, peeled; Kosher salt and freshly ground black pepper; 2 tablespoons (30ml) vegetable oil, beef fat, schmaltz, or duck fat (see note); 4 tablespoons (55g) unsalted butter; 3 medium garlic cloves (15g), peeled and smashed; 1 tablespoon fresh thyme leaves, chopped; Flaky sea salt, such as Maldon, for finishing (optional)