Preheat the oven to 200C/400F/Gas 6 and dust a large baking tray with flour.
Mix the flour, salt and yeast in a large bowl and mix until well combined. Make a well in the centre of the mixture and steadily pour in the water until the dough is loose and easy to knead, but not too sticky.
Knead the dough for 10 minutes on a lightly floured work surface, or for 5 minutes in an electric mixer fitted with a dough hook.
Add the thyme and knead for a further 30 seconds, or until it is well combined. Put half the chorizo in the middle of the dough and then fold the edges around it to cover and knead it for a further minute.
Place the dough mixture on a dusted work surface. Shape it into a ball, making sure the top is taught and smooth. Using a rolling pin, roll it out into a rough oval shape.
Cut slashes in the loaf to look like a fern leaf with a knife or razor blade, and then open the slashes apart with floured hands.
Push the remaining chorizo into the top of the dough, then cover loosely with oiled cling film (you may need several pieces). Leave to rise in a warm place until the dough has doubled in size, approximately for one hour.
Once risen, remove the clingfilm, brush the dough with milk and place in the hot oven. Spray the oven with water before closing - this will prevent a crust from forming too quickly and will help to ensure a good rise. Bake for 30–35 minutes, or until the bread is well-risen and golden brown. It will come off the tray once fully cooked too.
To serve, either keep the bread whole or cut into chunks to eat before or with a main meal.