Slice the baguette in half lengthways and hollow out (make crumbs from the bread centre and save for another recipe). Spread the bottom half with the pesto. Slice the tomato and layer it over the pesto. Slice the mozzarella and add in a layer over the tomato.
Finish with layers of spinach and basil, plus the salami, folded in half if necessary to fit the width of the baguette.
Wrap in baking parchment, tie with string and pop in the fridge weighted under something heavy (we used a hefty griddle pan). Leave for at least 1 hr (or overnight if you like). The flavours will mingle and the sandwich will flatten down, making it a doddle to cut up without all the ingredients falling out all over your picnic rug.