3 pounds (1.4kg) sweet potatoes, peeled and cut into 1/2-inch chunks
3 tablespoons (45ml) vegetable or canola oil
Kosher salt and freshly ground black pepper
1/2 packet unflavored gelatin (about 1 1/4 teaspoons; 4g)
1/4 cup (60ml) homemade or store-bought low-sodium chicken stock
1 stick unsalted butter (8 tablespoons; 115g)
3 rosemary sprigs
1 teaspoon (5ml) fresh juice from 1 lemon
Place sweet potatoes in a large saucepan and cover with cold water. Heat water to 160°F (70°C) as registered on an instant-read thermometer. Cover and set aside for 1 hour.
Adjust oven racks to upper-middle and lower-middle positions and preheat oven to 400°F (200°C). Drain sweet potatoes and transfer to a large bowl. Toss with oil and season to taste with salt and pepper. Spread sweet potatoes on 2 rimmed baking sheets and roast until bottom sides are browned, about 30 minutes. Carefully flip potatoes with a thin spatula and roast until second side is browned and potatoes are tender, about 20 minutes longer.
Meanwhile, in a small container, sprinkle gelatin over chicken stock and let bloom for 5 minutes. Microwave until gelatin is dissolved, stopping and stirring as needed to help it dissolve, about 2 minutes.
In a small saucepan, melt butter over medium-high heat until foamy. Continue to cook, swirling pan, until milk solids in butter turn a deep hazelnut brown, 1 to 2 minutes. Remove from heat and add rosemary (don’t be alarmed; butter will foam and fizzle when rosemary is added), swirling to coat.
Add chicken stock to butter, return to medium heat, and bring to a simmer. Cook, swirling, until stock reduces slightly and sauce becomes rich and silky, about 3 minutes. Remove from heat, discard rosemary, and add lemon juice. Season with salt.
In a mixing bowl, toss potatoes with butter sauce and serve. Potatoes can be made in advance, tossed with sauce, and set aside until ready to serve; rewarm in the oven before serving.