Creamy Mashed Potato Soup with Dashi

Dashi and soy sauce add umami to buttery leftover mashed potatoes in this cozy soup from chef Shota Nakajima. As a kid, whenever he was hungry, he’d grab anything left over from the previous meal to make soup or fried rice. “I grew up in a Japanese family, and I was taught to never waste anything,” Nakajima says. This soup is his riff on a recipe from his mother. Add milk for a creamier texture, or more dashi for a thinner consistency. Substitute any mashed vegetable you have on hand, such a cauliflower or sweet potato, but be sure to finish the soup with generous pats of melting butter and an extra drizzle of soy.

Creamy Mashed Potato Soup with Dashi
Photo: Photo by Victor Protasio / Food Styling by Torie Cox / Prop Styling by Claire Spollen
Active Time:
20 mins
Total Time:
4 hrs


  • 1 medium-size yellow onion, finely chopped

  • 2 large celery stalks, finely chopped

  • 2 1/2 to 3 1/2 cups dashi or chicken stock

  • 3 cups leftover mashed potatoes

  • 1/4 cup whole milk (optional)

  • 1/2 teaspoon kosher salt, or to taste

  • 1/4 teaspoon black pepper, or to taste

  • Unsalted butter and soy sauce, for serving

  • Chopped mitsuba or celery leaves, for garnish


  1. Combine onion, celery, and 2 1/2 cups dashi in a small Dutch oven. Bring to a boil over medium-high. Reduce heat to medium-low, and simmer, uncovered, stirring occasionally, until vegetables are tender, 35 to 40 minutes.

  2. Add leftover mashed potatoes to dashi mixture, and whisk until well combined. Stir in milk, if desired, for a creamier texture. Stir in remaining 1 cup dashi, 1/4 cup at a time, to thin soup to desired consistency. Cook over medium-low, stirring occasionally, until warm, 4 to 6 minutes. Season with salt and pepper.

  3. Top servings with butter and drizzle with soy sauce. Garnish with mitsuba or celery leaves.

Make Ahead

Soup can be prepared up to 3 days in advance and stored in an airtight container in refrigerator. Reheat over low, stirring often to prevent simmering.

Suggested Pairing

Aromatic dry Riesling.

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