1.In a medium bowl, gently toss together tempeh, tamari, mirin and garlic; set aside to let marinate for 30 minutes.
2.Meanwhile, arrange squash halves, cut sides down, in a large baking dish.
3.Pour 1/2 cup water into dish and bake until just tender, 30 to 45 minutes. Remove from oven, turn over squash and set aside to let cool slightly.
4.Heat oil in a large skillet over medium high heat. Drain tempeh, then add it to the skillet and cook, stirring occasionally, until golden brown, 7 to 8 minutes; transfer to a plate and keep warm.
5.Meanwhile, heat pasta sauce in a medium pot over medium heat until hot throughout then add broccoli and cook until just tender, about 5 minutes. Stir in spinach and remove pot from the heat.
6.Using a fork, scrape strands of spaghetti squash onto a serving platter. Spoon hot pasta sauce and broccoli over squash, top with tempeh and serve.