Chop the kale into rough 1-inch pieces, including the ribs. Roughly chop the tomatoes. (If desired, reserve about 1/4 cup fresh tomato pieces for garnish.) Peel and dice the onion.
Heat the oil in a large, deep pot, or a large wok. When it is hot, add the onion and cook for about 8 minutes over medium-high heat, stirring frequently. When the onion is getting soft, stir in the cumin, coriander, and turmeric. Stir in the tomatoes and cook for about 2 minutes.
Add the greens one handful at a time, stirring constantly to coat them with the onions, oil, and spices. When they have all been added, sprinkle the salt and a generous amount of fresh pepper over them and stir.
Pour in 1/2 cup water. Cover the pot and turn the heat down to medium. Cook for 5 to 10 minutes, or until the greens are tender to your taste.
Remove the lid, turn off the heat, and toss the greens with the lemon juice. Serve hot, garnished with extra tomato, if desired.