Preheat the oven to 200 C, 180 Fan, Gas 6. Find an ovenproof baking dish 25 cm x 15 cm.
Cut the sweet potato into rough 4 cm cubes and cut the Desiree into 2.5 cm cubes. Halve the red onion and cut each half into 4-6 segments discarding any tough outer skin.
Deseed the peppers and cut into 2.5cm squares and separate the cloves of garlic. Put everything into a large roasting tin and, using your hands, give the vegetables a good coating of olive oil. Season with black pepper but no salt.
Cook for 45 minutes or until the vegetables are cooked through and tinged with brown here and there.
Turn up the oven or put on the grill. Put the thinly sliced cheese on top of the bake and cook until the cheese has melted and turned slightly brown on top- about 5-10 minutes.