1. Fill a large pan with water and add a pinch of salt. Scrub and peel the potatoes, then chop them into small, equal-sized chunks. Slide the potatoes into the pan and bring to the boil. Simmer for 15-20 mins, till you can crush them with a fork.
2. Meanwhile, fill your kettle and boil it. Snip the semi dried tomatoes into a heatproof jug and pour in 200ml boiling water. Set aside to soak for 10-15 mins.
3. While the semi dried tomatoes soak, trim, peel and finely chop the onion. Trim, peel and finely chop the carrots. Trim the dry ends off the celery, and finely slice it. Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the chopped onions, carrots and celery. Fry for 5-6 mins, stirring a few times, till the veg start to brown.
4. While the veg fry, thickly slice the portobello mushrooms. Peel and grate or crush the garlic. Stir the mushrooms and garlic into the veg. Season with a pinch of salt and pepper and fry, stirring often, for 5 mins till the mushrooms are juicy.
5. Drop the thyme sprigs into the pan. Drain and rinse the lentils, then add them too. Add the tomato ketchup. Pour in the semi dried tomatoes and their stock. Bring to the boil, then turn the heat down and cover. Simmer for 15 mins, stirring once or twice, till the sauce thickens a little.
6. The potatoes should be tender by now. Drain them, then tip them back into the pan. Add 1 tbsp olive oil and a pinch of salt and pepper. Mash the potatoes together till smooth.
7. Heat your grill to medium-high. Fish the thyme sprigs out of the lentils and taste the sauce. Add a pinch more salt or pepper, if you think it needs it. If your frying pan is suitable for sliding under the grill, then leave the lentil mix in there. Otherwise, spoon the mushroom and lentil mix into a heatproof dish. Spoon over the mash.
8. Slide the pan or dish under the grill and cook for 6-8 mins, till bubbling and golden. Cool for a few mins before serving.