Prepare the vegetables. Chop the onions finely. Chop the garlic as well. Cut the peppers into thin strips. Halve the leeks lengthwise, if very thick, and cut the halves into thin half rounds. Set aside.
In the meantime, bring the vegetable stock used for cooking the buckwheat groats to a boil. When the liquid boils, add the rinsed buckwheat groats, cover the saucepan with a tight-fitting lid and simmer on low heat until done to your liking. Check the buckwheat's package instructions regarding the cooking times, they can differ from pack to pack and sometimes a lot. Check the buckwheat 2-3 minutes earlier than indicated on the package, just to make sure you don't overcook it. It should be soft but still, have a good bite. Mushy buckwheat is not so good.
Heat the oil in a larger and deeper pan. Cook the onions for about 2 minutes. Add the garlic, peppers, and leeks. Stir well and cook for about 3-4 minutes or until slightly softer.
In the meantime, halve the cherry tomatoes. Add them to the pan together with the smoked and sweet paprika powder, turmeric, chili flakes, lemon juice, and vegetable broth.
Cover, turn the heat down to medium-low, and cook the vegetables, stirring occasionally, for about 10 minutes or until done to your liking.
Add the cooked kasha to the vegetable pan and stir well but carefully.
Adjust the taste generously with sea salt, freshly ground black pepper, and the remaining lemon juice.