Rinse the lentils in a fine-mesh sieve under cold water for about 1 minute, until the water runs mostly clear. Dice the 150 g onion, mince the garlic and grate the ginger. Slice the 2 onions for frying 3 mm thick and set them aside.

Melt 1 tbsp ghee in a 5-quart / 4.7 L heavy pot over medium heat. Cook the diced onion for 5 minutes until soft and translucent. Add the garlic, ginger, tomato paste, turmeric, cumin and coriander, and stir for 1 minute until fragrant and the paste darkens a shade.

Add the lentils, water and 1 1/2 tsp salt. Bring to a boil, skim off the foam, then reduce to low and partly cover. Simmer for 20 minutes, stirring every 5 minutes, then stir in the spinach and cook 5 minutes more, until the lentils have collapsed and the dal looks creamy.

Toss the sliced onions with the flour and 1/4 tsp salt in a bowl until every strand is coated. Heat the oil in a 30 cm / 12-inch skillet over medium-high heat for about 2 minutes, until a single slice sizzles right away. Fry the onions for 10 to 12 minutes, stirring every minute, until deep golden. Lift them out with a slotted spoon onto a paper towel-lined plate. They keep darkening as they cool, so pull them before they look brown.

Stir the lemon juice into the dal and taste for salt. It should fall off the spoon in a slow ribbon. If it's stiff, stir in 60 ml hot water.

Heat 1 tbsp ghee in a 16 cm / 6-inch skillet over medium heat for 1 minute. Add the cumin seeds and pepper flakes and let them sizzle for about 1 minute, until the seeds darken a shade and smell toasty. Pour the hot ghee straight into the dal and stir once.

Ladle about 450 g of dal into each of 4 bowls. Top each with 125 g cold Greek yogurt, a quarter of the crispy onions and chopped cilantro. Serve within 10 minutes so the onions stay crisp.