Place a large pot over high heat and add remaining 1 tablespoon oil and onion. Cook, stirring often, for about 2 minutes. Add bell pepper and garlic, and stir until very lightly browned. Add reserved diced eggplant and stir until tender, 5–7 minutes. Stir in ½ teaspoon salt and ¼ teaspoon pepper, then tomato purée and oregano. Reduce heat to medium-low, cover, and cook, stirring often, until all vegetables are tender. Add cooked buckwheat; stir to combine and warm through.