Heat the oil in a saucepan. Add the onion, celery, and carrot, and cook gently for about 15 minutes until soft. Add the garlic and cook for another minute.
Add the lentils, stock, tomatoes, and bay leaf. Bring to the boil, reduce the heat, then half-cover the pan with a lid and simmer for 25€“30 minutes until the lentils and vegetables are very soft. Add more water if needed to ensure the lentils are covered.
Add salt and pepper to taste. Stir in the kale so that it wilts that take off the heat and let cool completely. Remove the bay leaf from the ragu.
Spread the ragu onto dehydrator trays lined with nonstick sheets or parchment paper. Dehydrate at 63C/145F for 8-10 hours until totally dry. Vac pac or store in baggies until ready to use.
Put the ragu into a pot and cover with water.
Place pot over medium heat and bring to a boil.
Turn down the heat and cook, stirring occasionally, for about 5-10 minutes, until fully rehydrated.
Top with parmesan cheese - you can bring a block and use your knife to grate it, or a small portable cheese grater (luxury). Alternatively, bring pre-shredded parmesan for ultimate convenience.