Preheat the oven to 180°C.
Cut the marrow in half (horizontally) then scoop out the seeds. Cut the flesh into 1cm half-moon pieces.
Cut the leek into 1cm slices.
Arrange the marrow pieces in a baking dish, scatter the leek and tomatoes around them.
Mix the oil with the garlic, lemon juice and zest. Add the chilli flakes, if used.
Pour this over the vegetables and season with salt and pepper.
Bake for about 20-25 minutes until soft and light brown.
Meanwhile, toast the pine nuts.
When the marrow is ready, take it out of the oven and transfer it to a big salad bowl. Fold in the rocket leaves then scatter the pine nuts on top.
Serve warm.