Heat a large saucepan on a low to medium heat, then add the onion and cook for 3-5 minutes. Add the garlic and cook for a further 2-3 minutes. Now add the butternut squash, diced chilli and tomato paste. Cook for about 5 minutes, stirring regularly.
Add the lentils and vegetable stock, bring to the boil then reduce to a low simmer. Leave to cook, uncovered, for 20-25 minutes until both the squash and lentils are fully cooked. Add the chilli flakes and black pepper to taste, then blend until smooth.