Heat the oil in a large pan over medium heat and add the chopped rosemary.
Next add in the garlic, onion, carrots and mushrooms and for about 10 mins, or until softened, stirring regularly.
Add the beef and lentils. Season with salt and pepper and cook for a further 5 mins, breaking up the meat with a spoon.
Add in in the chopped tomatoes, along with 1 tin worth of water.
Bring to the boil, lower the heat, and simmer for 30 mins, or until the sauce is thickened and reduced.
Season with salt and pepper, to taste, and serve straight away with spaghetti, other pasta or zoodles.
You can also freeze the Bolognese for up to 3 months.