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Ask an adult to slice the ciabatta loaf in half lengthways and heat the oven to 200C/180C fan/gas 6.
Put the ciabatta loaf halves, crust-side down, on a large baking tray and drizzle with a little olive oil. Pop them in the oven for a few mins until just golden and lightly toasted.
Put the olive oil, balsamic vinegar, garlic and mustard in a bowl, then whisk them together with a fork.
Remove the toasted ciabatta halves from the tray and drizzle the bottom slice with about half of the dressing.
Arrange the rest of the ingredients in layers. Start with a large handful of baby spinach, then a few artichoke hearts.
Next add the slices of pepper, the prosciutto, basil, mozzarella and, finally, the red onion.
Drizzle over the rest of the dressing and pop the other slice of ciabatta on top.
Press down on the sandwich to squash all the layers together.
Wrap the sandwich in baking parchment and tie it together with a couple of pieces of string.
Place a heavy baking tray on top of your sandwich and top it with weights or loaf tins filled with baking beans. Pop it all in the fridge overnight or until you are ready to eat it. Cut and serve in slices for the perfect picnic snack.