Put the cooked buckwheat in a mixing bowl and loosen the grains with a fork.
Thinly slice the fennel and onion, and finely chop the garlic.
Heat the olive oil in a frying pan over a medium heat. Add the fennel and onion and gently sauté for about 5–8 minutes, until softened. Add the garlic and cook for another minute.
Stir in the dried herbs and vegetable stock. Simmer for about 10 minutes, until the fennel is tender and most of the liquid has reduced.
Add the drained lentils and lemon juice, then fold in the cooked buckwheat. Heat through, stirring gently.
Remove from the heat and stir in the fresh herbs. Add the crumbled feta, if using.
Taste and adjust the seasoning with salt, pepper or a little extra lemon juice. Serve warm or at room temperature.