Heat the oil in a saucepan over a medium heat, add the onion with a big pinch of salt and cook until softened, around 5 mins. Add the oregano, garlic and basil and cook for 1 min longer, then tip in the plum tomatoes. Fill the can halfway up with water and add to the pan. Leave to simmer for 10 mins, then blitz with a hand blender. Season.
Meanwhile, heat the grill to high. Spread the toast with Marmite, then top with grated parmesan, put onto a baking sheet and grill for 3-5 mins until golden. Spoon the soup into two bowls and top with the slices of cheesy Marmite toast. Grind over some black pepper to serve.