Heat the oven to 200°C/400°F. In a saucepan heat the olive oil over a medium heat. Add the onion and plenty of salt and pepper. Cook for 5 minutes until softened. Add the garlic and chilli flakes, cook for another minute. Add the tomato puree and anchovies, cook for a couple of minutes until the anchovies have melted into the tomato puree.
Add the chopped tomatoes, stock and thyme to the pot. Stir through the cherry tomatoes followed by the lentils and barley. Pour the mixture into a baking dish (it will be watery at this point). Put the dish in the oven and bake for 40-50 minutes until the lentils and barley are tender.
Cut the feta into rectangles and then cut in half lengthways so that you have slabs of the cheese. Remove the dish from the oven and set the oven to grill (broil) at a high temperature.
Nutrition information is an estimate and will vary depending on the exact ingredients used.