Peel and finely chop the onion. Trim, peel and finely chop the carrot into small dice. Thickly slice the mushrooms. Set the veg aside.
Pour 1 tbsp oil into a large pan or casserole dish and bring it to a medium-high heat. Rub a good pinch of salt and pepper into the chicken breasts, then carefully add them to the pan. Cook for 2-3 mins on each side, till golden brown. Remove from the pan and set aside on a plate.
Keep the pan on the heat and add the onion, carrot and mushrooms. Season with a pinch of salt and pepper. Cook for 5 mins, stirring frequently, till golden brown and starting to soften.
While the veg fry, peel and crush or finely chop the garlic. Pick the thyme leaves off their sprigs. Stir in the garlic and thyme leaves into the veg and cook, stirring, for 1 min.
Drain the lentils and rinse them under a cold tap. Tip them into the pan and pour in 500ml boiling water. Crumble in the stock cube and stir well to dissolve it. Return the chicken breasts to the pan, making sure that they are submerged in the liquid. Pop a lid on the pan. Bring to the boil, then reduce to a simmer and cook for 15 mins, or till the chicken is cooked through.
Remove the pan from the heat and stir in 100ml of the cream. Taste the sauce and add more salt and pepper if needed. Ladle into warm bowls and serve straight away.