Rinse buckwheat groats. Bring a saucepan of water to a boil, sprinkle in the buckwheat groats, and simmer until buckwheat is tender, about 10 minutes. Drain and cool.

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Heat olive oil in a skillet over medium heat; cook and stir onions and garlic until onion is translucent, 5 to 8 minutes. Set aside to cool.

Lightly toss cucumber, parsley, mint, lemon juice, and mixed herbs in a large salad bowl until thoroughly combined; stir in cooked buckwheat and onion mixture.