Cut the cardoons into small pieces and cover with cold water and the lemon juice.
In a dutch oven, combine the pancetta and onion and cook together over medium-low heat until the pancetta gives up its fat and the onion begins to soften and get browned. Add the lamb and season with salt and pepper. Sprinkle the flour over the meat and stir over medium heat for three minutes, then stir in the pepper flakes, and continue to cook and stir until the meat is browned.
After 30 minutes, stir in the tomato paste, add the parsley, garlic, and thyme, and stir well. Add 1 quart of water, add the cardoons, season with salt and pepper, and cook 30 minutes. Serve hot.