Storage notes: Keeps for 3 days when refrigerated or up to 2 months when frozen
Level of difficulty: Easy
Special requirements: Pressure cooker
1. Sauté the sweet onions in the base of a pressure cooker over high heat until they turn light golden, about 10 minutes.
2. Add the water and sweet potato, and pressure-cook at a gauge pressure of 1 bar/15 psi for 25 minutes. Start timing when full pressure is reached.
3. Let the cooker cool, or run tepid water over the rim, to depressurize it.
4. Strain the stock through a fine sieve. Discard the solids.
5. Cool the stock and reserve 635 g for the soup.
6. Melt in the base of a pressure cooker over medium heat.
7. Add to the melted butter, and stir to combine.
8. Pressure cook at a gauge pressure of 1 bar/15 psi for 20 minutes. Start timing when full pressure is reached.
9. Depressurize the cooker quickly by running tepid water over the rim.
10. Blend into the sweet potato mixture using an immersion blender until smooth. Add more stock if necessary to thin the soup to a desired consistency.
11. Adjust the seasoning with salt and pepper.
12. Crush kaffir lime leaves into the soup, and let them infuse for about 15 minutes.
13. Remove the leaves from the soup.
14. Roast the sweet potatos and chestnuts at 175°C/350°F for about 45 minutes, and then let them cool. Cut the potatos into cubes, and slice the chestnuts thinly.
15. Toast the marshmallows with a blowtorch.
16. Divide the garnishes into six soup bowls.
17. Reheat the soup, pour around the garnish, and serve warm.
Modernist Cuisine: The Art and Science of Cooking by Nathan Myhrvold, Chris Young and Maxime Bilet, The Cooking Lab, 2011.
Chef Sean Brock, author of Heritage, grew up in a town where seed saving was a way of life. "You just saved these seeds not because you were poor, but because you really loved the flavor of a particular tomato or a particular bean," he says.