Preheat the oven to 400˚F. Butter a 8 x 11-inch dish. Spread bread cubes on a rimmed baking sheet and bake for 20 minutes, or until golden brown. Remove from the oven and set aside in a large mixing bowl.
Meanwhile, heat olive oil and butter in a large skillet over medium-heat. Once the butter has melted, add onion, celery, green onions, thyme, salt, and pepper. Cook, stirring occasionally, for 8 to 10 minutes, or until the vegetables have softened. Transfer the vegetables to the bowl with the toasted bread.
Add parsley and sage to the bowl, then slowly stir in the vegetable stock, letting it absorb as you mix the stuffing. Once most of the liquid has been absorbed by the bread, stir in soy sauce.
Transfer the stuffing mixture to the prepared baking pan and bake at 400˚F for 30 minutes. Garnish with the green onions and serve immediately.
Tip: If you want to skip toasting the sourdough bread, open the bag slightly and let it dry out for at least 48 hours. The bread should be stale/hard without the use of the oven.