Put three-quarters of the chickpeas in a food processor with 3 tbsp olive oil, garlic and half a cupful of water. Blitz until smooth, adding a little more water if necessary to make a thick cream consistency. Add the rest of the chickpeas, then whizz again for a few secs until nobbly. Season with salt and a good squeeze of lemon juice.
Mix together the harissa and tomato purée with the rest of the olive oil. Drizzle over the houmous to serve.
Heat oven to 190C/170C fan/gas 5. Split 6 pittas horizontally into two circles, then cut each into 6 triangles. Lay them in a single layer, rough side up, on 2 or 3 baking sheets. Use a pastry brush to brush the pitta triangles with olive oil, then sprinkle with fennel seeds and a pinch of sea salt. Bake for 10-15 mins, until golden. Leave to cool on the tray.
I like to use Belazu Rose Harissa (in jars from delis and supermarkets) which is relatively mild but with a complex flavour. Some tubes of North African harissa can be explosively hot – use extra tomato purée and olive oil to lighten.