Heat the oil in a large saucepan, add the onion and red pepper and cook over a medium heat for 3 minutes until softened. Add the mustard and fennel seeds and cook for 30 seconds until starting to pop and smell fragrant. Add the garlic, ginger, garam masala, cumin, coriander, turmeric and chilli flakes and cook, stirring, for 30 seconds.
Add the butternut squash and cauliflower and stir to coat in the spices. Stir in the tomatoes and measurement water, season with salt and pepper and bring to the boil, then cover and simmer for 10 minutes, stirring occasionally, until the vegetables are tender. Add the peas and cook for a further 3 minutes.
Stir in half the cashew nuts and sprinkle the rest on the top to garnish. Serve with naan bread.