• Yield: Six servings (1.3 kg/6 cups)

  • Time: 1 3/4 hours overall, including 45 minutes of preparation and 1 hour unattended total

Storage notes: Keeps for 3 days when refrigerated or up to 2 months when frozen
Level of difficulty: Easy
Special requirements: Pressure cooker

  • Water: 660 g (weight), 100% (scaling)

  • Sweet potato, thinly sliced: 600 g, 91%

  • Sweet onion, thinly sliced: 150 g, 23%

  • Neutral-tasting oil: 25 g, 4%

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.

  • Unsalted butter: 113 g, 17%

6. Melt in the base of a pressure cooker over medium heat.

  • Sweet potato, peeled and diced: 500 g, 100%

  • Baking soda: 2.5 g, .5%

  • Salt: to taste

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.

  • Sweet potato stock, from above: 635 g, 127%

  • Black peppercorns, ground: to taste

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.

  • Salt: to taste

  • Black peppercorns, ground: to taste

  • Makrud (kaffir) lime leaves, crushed: 2-3 leaves

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.

  • Roasted sweet potato confit: 18 pieces

  • Roasted purple sweet potato confit: 12 pieces

  • Toasted marshmallows, small: 12 pieces

  • Roasted chestnuts, thinly sliced: 12 pieces

  • Fried sweet potato: as needed

  • Fried sage: as needed

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.