Peel and finely chop the onion. Trim, peel and dice the carrots. Halve the pepper and scoop out the seeds and pith. Dice it to match the carrots.
Place a pan on a medium heat. Add 2 tbsp olive oil, the chopped veg and a pinch of salt and pepper. Fry, stirring often, for 8 mins till glossy and starting to soften.
Meanwhile, peel and crush the garlic. Drain and rinse the lentils. Break the spaghetti into 2cm-long pieces. Fill and boil the kettle.
When the veg have cooked for 8 mins, stir in the garlic and cook for 1 min. Stir in ½ tsp harissa paste (it’s spicy, so you can start with a little and add more later if you like). Pour in the chopped tomatoes and 600ml hot water from the kettle.
Bring to a bubble. Add the lentils and spaghetti with a pinch more salt and pepper. Pop the lid on. Simmer for 8 mins till the spaghetti is just cooked.
Meanwhile, trim any dry ends from the cavolo nero and shred the leaves as finely as possible. When the minestrone has cooked for 8 mins, add the cavolo nero and cook for 2-3 mins to slightly wilt it.
Taste the soup and add more salt, pepper or harissa paste if you think it needs it. Ladle the soup into warmed bowls and top each with 1 tbsp yogurt to serve.