Heat the oven to 325 degrees. In a heavy, ovenproof Dutch oven or saucepan with a tight-fitting lid heat 1 tablespoon of oil over medium-high heat. Sprinkle the lamb with salt, and when the oil is hot brown half of the cubes on all sides, about 5 minutes. Remove the lamb to a plate with a slotted spoon and add more oil if necessary. Brown the rest of the lamb and then remove that from the pot as well.
Turn down the heat to medium low and add the onions, carrots and garlic to the pot. Cook for 3 to 5 minutes, until the vegetables begin to soften. (Make sure not to let the garlic color too much.)
Add the chopped tomatoes, stock, herbs and the browned lamb, along with any juices that have accumulated. Bring the stew to a boil over high heat, stirring gently with a wooden spoon to get up all the brown bits from the bottom of the pot.
When the stew comes to a boil, cover the pot and put it in the oven. Cook until the lamb is just tender, 2 to 3 hours.
Stir the butternut squash into the stew, re-cover it and return the pot to the oven until the squash is tender, another 20 to 25 minutes.