Fry the pancetta in a dry frying pan until it begins to crisp. Remove from the pan and put to one side to cool slightly.
Tip the bread mix into a bowl and add the pancetta, thyme, parmesan, oil or water. Mix to a dough using palette knife, adding a little more water if the dough feels dry, and turn out onto a floured surface. Knead the dough by hand, or with the dough hook of a free-standing mixer, for 10 minutes until it feels smooth and elastic. Turn into a large, lightly oiled bowl, cover with cling film and leave to rise in a warm place until doubled in size (about 1-1½ hours).
Grease a large baking sheet. Roll out the dough to a 33 cm (13in) square and scatter with an even layer of Gruyere, peppers and chilli. Roll up loosely and transfer to the baking sheet with the join underneath. Pinch the ends together to seal. Cover loosely with oiled cling film and leave to rise in a warm place for 1 hour until it is about half the size again. Preheat the oven to Gas7, 220°C , fan 200°C.
Using a sharp floured knife, make deep cuts into the loaf to mark it into 8 portions. Brush the dough with milk and scatter with thyme sprigs. Bake for about 35 minutes until golden, covering with foil if it gets too dark. Transfer to a cooling rack, then wrap in foil to transport.