Preheat the oven to 400°F (200°C).
Using a sharp knife, poke several holes in the spaghetti squash skin to allow steam to escape. Microwave the whole squash on high for 5 minutes to soften slightly (optional, skip if not using a microwave).
Carefully cut the squash lengthwise into four pieces. Scoop out and discard the seeds.
Brush the cut surfaces with 1 teaspoon olive oil and sprinkle with 1/2 teaspoon Italian herb blend. Place the squash pieces cut-side down on a baking sheet sprayed with nonstick spray.
Roast in the preheated oven until the flesh is tender, about 45 minutes (longer if you did not microwave first). Remove and let cool until you can handle it.
While the squash roasts, prepare the vegetables: drain and chop the artichoke hearts, halve the cherry tomatoes and olives, slice the mushrooms, chop the onion and zucchini, thinly slice the green onions, and crumble the feta.
Mix the broth and vinegar together in a small bowl and set aside.
When the squash is cool enough to handle, use a fork to shred the flesh into strands and discard the skins; keep the shredded squash aside.
Heat 2 teaspoons olive oil in a large nonstick skillet over medium-high heat. Add the mushrooms and cook until browned and their liquid has evaporated, about 4–6 minutes. Transfer mushrooms to a plate.
Add the remaining 2 teaspoons olive oil to the skillet, then sauté the chopped onion and zucchini over medium-high heat until lightly browned, about 5 minutes.
Add the minced garlic and 1/2 teaspoon Italian herb blend and cook another 1–2 minutes until fragrant.
Pour in the broth-vinegar mixture, reduce heat to medium, and cook about 2 minutes. Return the cooked mushrooms to the pan.
Add the cherry tomatoes, olives, and artichoke hearts and cook just until warmed through, about 1–2 minutes.
Gently fold the shredded spaghetti squash into the vegetable mixture until combined and heated through.
Stir in 3/4 cup crumbled feta. Season to taste with salt and fresh ground black pepper.
Transfer to a serving dish, sprinkle with remaining feta and sliced green onions, and serve hot.