Trim the lamb of any large areas of fat, and season with salt and pepper. Dredge with flour, shaking off the excess. Heat two tablespoons of olive oil in a large Dutch oven over medium-high heat. Add lamb in a single layer, in multiple batches if needed to avoid over-crowding the pot. Sear until brown on all sides, about five minutes. Transfer to a plate and set aside.

Add additional olive oil to the pot, and add the onions, cooking until they are golden, about five minutes. Add mushrooms, season with salt to taste, and cook until well browned, about five minutes.

Add red wine, scraping up any browned bits from the bottom of the pot. Cook until the wine is reduced by about half, approximately five minutes. Add reserved lamb, beef stock, chestnuts, herbs and tomatoes. Bring the mixture to a boil. Reduce heat to medium-low and simmer for two hours. Combine gelatine with 1/4 cold water and stir to combine. Add mixture to the pot and cook until the body is silkier and slightly thicker, about 5–10 minutes. Add cranberries and cook for two minutes. Serve over a bowl of creamy polenta.