Heat the oil in a wide, heavy-bottomed pot over medium high heat. Sauté the onion for 3-5 minutes, or until translucent. Add the garlic and carrots and fry for another couple minutes. The carrots should start to soften. Toss in the cubed potatoes, curry powder, salt and pepper. Gently toss together so that all the vegetables are coated in the spices. Add the lentils and stock and bring to a simmer. Turn the heat to low and cover, leaving the lid slightly ajar. Cook for 30 minutes.
Pierce the potatoes with a knife to check they are fully cooked. Taste for seasoning and adjust to your taste. Stir in the spinach and gently cook for about 2 minutes or until you see the spinach has started to wilt.
When serving, I like to garnish each plate of curried lentils with a dollop of greek yogurt, drizzle of olive oil and a twist of black pepper. The contrast between the dark lentils and the white yogurt is very pretty.