Lentil Soup With Harissa & Preserved Lemons

'There's harissa for gentle heat, nutty lentils, tender broccolini and chard, and lemon to perk everything up,' says the author of the new vegetarian cookbook Dinner.

- MEERA SODHA

'I love to serve it with fresh herbs, cold yogurt and olive oil; but really, the key to a good soup is to customize it as you wish'

3/2 Tbsp. extra-virgin olive oil, plus more for serving

1 large (10-oz.) yellow onion, finely chopped (2 cups)

2 celery stalks, finely chopped

4 garlic cloves, crushed

1 preserved lemon (from 1 [7-oz.] jar), seeded and finely chopped