Heat the 3 tablespoons oil in a large, high-sided frying pan with a lid over medium-high heat. Once shimmering, add 1/2 teaspoon mustard seed, 1/2 teaspoon cumin seed and 2 dried chiles. As soon as mustard seeds begin to pop (a few seconds) add 1 pound quartered new potatoes.
Reduce heat to medium and fry potatoes, stirring occasionally, until they start to brown a little. Add the 1 teaspoon cumin, 1 teaspoon coriander, 1 teaspoon sugar, 1/4 teaspoon cayenne, and 1/4 teaspoon turmeric. Stir to coat potatoes in the spices.
Add the 1 pound chopped green beans and 2 cloves sliced garlic and cook a few minutes more. Add 1 cup vegetable broth and 1 teaspoon salt, bring to a boil, then reduce heat to low. Cover and cook for 10 minutes.
Add the 3 sliced tomatoes, remaining 1/2 teaspoon salt and drained can of chickpeas and simmer, uncovered for another 10 minutes. Taste and add more salt if desired. Serve with lemon wedges and steamed rice.