Preheat the oven to 240°C/475°F/gas 9 (full whack). Put a large non-stick, ovenproof frying pan on a medium-high heat. Squeeze the sausage meat out of the skins directly into the frying pan, with ½ a tablespoon of olive oil. Use a wooden spoon to roughly break up the meat. Add the oregano and fennel seeds (if using), and stir-fry until the sausage meat is lightly golden. Peel and finely slice the onions then add to the pan with a pinch of sea salt and black pepper. Add the jalapeños and a splash of their vinegar. Cook on a medium heat for 5 to 10 minutes, or until the onions are soft and caramelised, stirring regularly. Meanwhile, for the dough, pile the flour into a bowl with a good pinch of sea salt. Add up to 250ml of water, mixing until it comes together as a smooth, elastic dough. You want it to be pliable. Knead the dough on a lightly floured surface for a couple of minutes, then form into a rough round. Now, tip your topping out of the pan for a moment, returning the pan to a medium heat. Place the dough in the pan and – being careful not to touch the hot pan – use your fingertips to stretch out the dough to cover the base. Spoon the topping back over the pizza, spreading it out and pushing the sausage into the dough, then grate over the cheese. Turn the heat back up to medium-high and cook for 3 to 5 minutes, or until the base has started to crisp up. Transfer to the oven for 10 minutes, or until golden, puffed up and sizzling. Serve with a nice salad, if you like.