Preheat the oven to 200°C/190°C fan/Gas 6. Place a baking tray in the oven to heat up.
Peel and finely chop the onion. Halve the pepper and scoop out the seeds and white pith. Finely chop it. Peel and grate the garlic.
Heat a large frying pan for 2 mins. When warm, drizzle in 1/2 tbsp olive oil. Add the onion and pepper and cook gently for 5-6 mins till just softened. Add the mince and garlic. Cook for 5 mins till the mince is browned.
Meanwhile, finely chop the tomatoes. Pop them in a food processor and blitz till smooth if you like. After the mince has cooked for 5 mins, add the tomatoes. Add a pinch of chilli flakes and season generously. Leave to simmer on a medium heat for 10 mins, till the sauce is thick.
While the mince is cooking, tip the flour into a bowl. Add the fennel seeds and a large pinch of salt. Pour in 1 tbsp olive oil and 2 tbsp cold water. Mix till the dough comes together then knead for a couple of mins till smooth.
Place the dough on to a sheet of baking paper. Roll the dough into a large thin circle- nearly as large as your baking tray if you can.
Grate the lemon zest into the mince. Taste and add more chilli flakes or seasoning if you think it needs it. Spoon the mince on to the dough and spread out, leaving a 1cm border. Drain the mozzarella. Tear it into small pieces and scatter it over the mince. Scatter over half the rocket.
Roll the dough into a cylinder - like a roly poly. Pinch the ends and edges together to seal. Remove the hot baking tray from the oven. Lift the baking paper with the stromboli on to the hot tray. Slide into the oven to cook for 20 mins. It should be golden and crispy.
Cut the stromboli into slices. Serve with the rocket tossed with a little lemon juice.