Preheat oven to 350°F. Place bread cubes on baking sheets and toast in the oven until crispy, about 10 minutes. Remove from oven and set aside to cool.
Place squash and sausages on another baking sheet and drizzle with 2 tablespoons of oil. Place in the oven and bake until squash is tender and sausages are cooked through. Remove from the oven and let rest for 5 minutes before slicing sausages.
Meanwhile, heat remaining oil in a large skillet over medium-high heat. Add leek, shallots and garlic, and sauté until tender and fragrant, 2-3 minutes.
In bowl A, combine stock, eggs, salt, pepper, and rosemary. Whisk to combine.
In (big) bowl B, place bread cubes, squash, sausages, and veggies.
Pour bowl A mixture over bowl B and toss to combine.
Transfer stuffing to a casserole or baking dish and cover with aluminum.
Place in the oven and bake for 30 minutes. Remove foil and continue to bake for another 30 minutes, until topping is crisp.