Use two forks to finely shred the ham, or chop into small cubes. Thinly slice the onion whites, then shred the greens finely and leave in ice-cold water.
Mix the sesame oil, hoisin, soy, tomato purée, honey and five-spice, then stir in the spring onion whites and ham.
Cut the cucumber into long thin slices. Mix the sriracha and vinegar with a good pinch of salt and the chilli flakes, and toss through the cucumbers to coat. If you can, leave for 15 minutes to marinate.
Split the baguette in half horizontally but not all the way through, and fill with the shredded ham, chilli cucumbers, drained spring onion greens, baby gem and mayo. Cut into 4 pieces to serve.