Pre-heat oven to 180°C / 350°F / Gas Mark 4.
Par-cook roots by boiling together for 5 minutes. Drain.
Lightly oil an ovenware dish. Layer potatoes and sweet potatoes with herbs and pepper, gently pressing down layers as you go.
Pour over stock and cover tightly with foil.
Bake for 40 minutes until vegetables are soft and set.
Remove foil, lay on sliced tomato and scatter with cheese.
Return to oven for 10 minutes until cheese is bubbling.