4 tablespoons butter
1 1/2 cups very finely chopped onion
1 tablespoon finely chopped garlic
1 tablespoon packed brown sugar
2 teaspoons kosher salt
2 teaspoons garam masala
2 teaspoons ground ginger
1/2 teaspoon ground coriander
1/2 teaspoon ground cinnamon
1/2 teaspoon whole yellow or brown mustard seeds
2 cups peeled, seeded, and finely chopped fresh tomatoes (see Recipe Notes)
1/2 cup vegetable or chicken stock
1 1/2 pounds whole green and/or yellow wax beans, ends trimmed and any tough strings removed
1/2 teaspoon ground grains of paradise (see Recipe Notes)
Finely grated zest and juice of 1 lemon
Melt the butter in a large saucepan over medium-high heat. Stir in the onion and cook until tender, about 5 minutes. Stir in the garlic, brown sugar, salt, garam masala, ginger, coriander, cinnamon, and mustard seed; cook, stirring, for 1 minute.
Stir in the tomatoes and stock. Bring to a simmer. Stir in the beans. Reduce the heat to low, cover, and cook gently in barely bubbling sauce, stirring occasionally, until the beans are fully tender, about 30 minutes.
Stir in the grains of paradise, lemon zest, and lemon juice. Serve warm, at room temperature, or lightly chilled.