Heat a large pan with 3 tbsp of oil. Add half of the sliced onions and fry for 4 minutes until softened.
In the meantime, in a large frying pan (you may need to use two frying pans or see Tip at the bottom of the recipe), heat 2 tbsp of oil. Tip in diced butternut squash and scatter the remaining sliced onions. Season with salt and pepper. Fry on medium heat for 15-20 minutes, stirring occasionally until butternut squash is soft, slightly browned and the onions are crispy.
Whilst butternut squash is cooking, continue preparing your dahl. To the frying pan with onions, add turmeric, chopped garlic and ginger and cook for another minute until fragrant.
Add rinsed red lentils, crumble in stock cubes, pour in a tin (400ml) of coconut milk, a tin (400g) of chopped tomatoes and 200ml of water. Mix well and cover with a lid. Simmer on low heat for 15-20 minutes until all the stock is absorbed and lentils are cooked. Add a splash of water if it gets too thick.
Take off the heat, stir in spinach and juice of ½ a lemon.
Serve dahl in bowls, topped with fried butternut squash cubes and crispy red onion. Garnish with a lemon wedge.