Peel the onion and carrot, dice them as finely as you can. Deseed and finely slice the red pepper. Wash 1 celery stick from the head of celery and dice that too. Peel and finely chop or crush 2 garlic cloves. Heat 2 tablespoons of oil in a saucepan and gently fry the onion, celery, carrots and pepper for 10 minutes, until beginning to soften. Add the garlic for the last 2 minutes.
Meanwhile, put the frying pan on the heat with a dash of oil. Season the lamb generously with salt and pepper and fry on a high heat for 6-8 minutes until nicely browned. Add the wine to the browned lamb and let it simmer for 1 minute. Tip the lamb into the onions, along with the passata, dried chilli flakes (use half if you don’t like too much heat), and dried mint. Top up with enough water to just cover the lamb and simmer gently for 30 minutes, adding a little more water if starting to dry out.
Meanwhile, tip 350ml of milk into the other saucepan and top up with another 300ml of water. Pop in a pinch of salt and bring it up to a gentle simmer. Tip in the polenta. Keep stirring while it thickens; it will start to gently sputter and pull away from the side of the pan as you stir. Heat gently for 5 minutes. Remove the polenta from the heat and beat in the butter and cheese. Taste and adjust the seasoning with salt and pepper. The polenta should be the consistency of loose porridge, so adjust the thickness by adding an extra dash of hot water if needed.
When ready, taste the lamb and adjust the seasoning with a little more salt and pepper. Put the polenta on a low heat to warm it. Divide the warm polenta between shallow bowls. Ladle over the rich, unctuous ragu and serve immediately.
When making the ragu I have advised browning the meat in a frying pan rather than adding it to the onions. Meat will rarely brown well when added to a pan of wet ingredients. This not only starts the cooking process but also adds flavour and colour from all those lightly caramelised edges. This is known as a Maillard reaction and accounts for the complex flavours that result from seared and roasted foods, as the sugars react at high heat.