1 large onion, quartered through root end
2 bay leaves
1½ C black beluga or French green lentils, rinsed, picked through
Kosher salt
¼ cup olive oil
1 t coriander seeds
½ t cumin seeds
3 T sherry vinegar or red wine vinegar
Freshly ground black pepper
6 radishes, trimmed, very thinly sliced
4 scallions, thinly sliced
1 C parsley and/or mint leaves
1 C thinly sliced celery hearts and leaves
Cook onion, bay leaves, and lentils in a large saucepan of simmering salted water until lentils are tender but still firm, 15–20 minutes. Drain; discard onion and bay leaves and transfer lentils to a medium bowl.
Heat oil in a small skillet over medium. Cook coriander seeds and cumin seeds, swirling skillet, until fragrant, about 1 minute. Add spice mixture and vinegar to lentils, season with salt and pepper, and toss to coat.
Just before serving, top lentils with radishes, scallions, herbs, and celery; season with salt and pepper.
Do Ahead: Lentils (without herbs and vegetables) can be marinated 3 days ahead. Cover and chill.