Preheat the oven to 180 degrees C.
Place the lentils in some water and bring to the boil. Simmer for 30 minutes until soft.
Meanwhile heat the oil and cook the chopped onion until soft. Add the crushed garlic and stir until fragrant.
Add the halved tomatoes and stir until they begin the break down. Once most of the tomatoes have released their contents add the drained lentils, sugar, vinegar and salt.
Slice the marrow into 5 cm slices and hollow it by spooning out and discarding the seeds. Replace the seeds with the lentil mixture and top with grated mature cheddar cheese.
Place in a preheated oven for 30 minutes, until the cheese is golden brown.
