Preheat the oven to 180°C.
Line an oven tray with baking paper, spread the sweet potato slices in a single layer on the tray and drizzle with a little olive oil.
Bake for about 15-20 minutes or until they are just starting to change colour and are almost cooked through.
In a large bowl mix together the tomato, crushed garlic and herbs, then add half the baked sweet potatoes slices and half the broccoli florets. Stir to combine.
Stir in the tuna and season to taste.
Transfer the tuna mixture to a medium-sized casserole (we used a 20 x 14 x 6cm ovenproof dish).
Arrange the remaining sweet potato and broccoli florets in a decorative pattern, scatter with grated cheese and bake for about 20 minutes or until the cheese is melted and golden and the dish is bubbling hot.
Serve with a green salad and if you don’t mind the carbs, crusty warm bread.