Slice the tops off the tomatoes and scoop out the seeds. In a small bowl, mix together the sardines, lemon zest and chilli. Stuff the tomatoes with the sardine mixture. Place the stuffed tomatoes on a small baking tray and drizzle over the oil. Bake for 4 to 5 minutes until the tomatoes just begin to soften. Serve on a bed of rocket leaves drizzled with olive oil.