In a saucepan, heat the olive oil over medium heat. Add the onion and saute until soft and translucent, about 4 minutes. Add the buckwheat groats, garlic, cumin seed, mustard seed and cardamom. Saute, stirring constantly, until the spices and garlic are fragrant and the buckwheat is lightly toasted, about 3 minutes.
Carefully pour in the stock. Bring to a boil, then reduce the heat to medium low, cover and simmer until the liquid is absorbed, about 10 minutes. Remove from the heat and let stand, covered, for 2 minutes.
Stir in the tomato and salt. Transfer to a serving bowl and sprinkle with the cilantro. Serve immediately.