Swiss chard and sweet potato curry with turmeric and ginger

piece of ginger, about 2 cm

smaller piece of fresh turmeric (or half a teaspoon of ground turmeric)

garlic

large sweet potato

About three or four large leaves of rainbow chard (or regular swiss chard)

vegetable oil

red lentils

mustard seeds

water or vegetable broth (in case of the latter, use less salt for seasoning)

Salt

few cilantro leaves (optional)

splash of lemon juice (optional)