Sweet potato & tofu curry soup

Tofu, extra firm, cut in half, pressed to drain for 20 minutes, cubed	1 pound
Butter, unsalted	3 tablespoons
Onion, yellow, sliced	1 small
Lemongrass, center white parts only, smashed, thinly sliced	1 stalk
Potatoes, Japanese sweet, peeled, cubed	2
Bell pepper, red, seeded, cut into thin strips	1
Coconut milk, light	30 ounces
Thai red curry paste	2 tablespoons
Fish sauce	1 tablespoon
Lime zest	1
Broth, chicken	½ cup
Cilantro, chopped	2 tablespoons

1. In a Dutch oven, melt the butter over medium heat. 
2. Add the onion, and sauté until it begins to soften, about 4 minutes. 
3. Add the lemongrass, potatoes, and bell pepper, and cook, stirring constantly, until the potatoes soften, about 10 minutes; transfer to a bowl. 
4. Add the coconut milk, curry paste, fish sauce, zest, and broth, and cook until the liquid begins to thicken, about 5 minutes. 
5. Add the potato mixture, and cook for another 10 minutes. 
6. Add the tofu, and cook, stirring well, for 3 minutes. 
7. Remove from heat, stir in the cilantro, and serve. 

Recipe adapted from Williams-Sonoma Soup of the Day by Kate McMillan.