Pommes de Terre Fondantes (Fondant Potatoes)

Ingredients

• homemade chicken or beef stock, or store-bought low-sodium chicken broth, plus e
• ounce unflavored gelatin, such as Knox
• pounds Yukon Gold potatoes, about 2 1/2 to 3 inches long each, peeled
• Kosher salt and freshly ground black pepper
• tablespoons vegetable oil, beef fat, schmaltz, or duck fat
• tablespoons unsalted butter
• medium garlic cloves , peeled and smashed
• tablespoon fresh thyme leaves, chopped
• Flaky sea salt, such as Maldon, for finishing