Place the tempeh in a shallow dish. Mix together the stock, tamari, onion powder, garlic powder, salt, and oregano and pour it over the tempeh. Marinate the tempeh for 2 hours or overnight.
Make the Noodles
In a large pot, cook the noodles according to the package. About three minutes before the noodles are done, add the greens to the pot. Drain the noodles and set aside until ready to use.
Make the Sauce
Place the vegetable stock, pumpkin, tamari, minced garlic cloves, garlic powder, onion powder, salt, black pepper, and sage in a sauce pan over meidum-high heat . Bring to a boil and then reduce heat to a simmer. Allow to thicken slightly, about 10 minutes.
Toss the cooked noodles and greens in with the sauce and keep warm.
Cook the Tempeh and Onions
While the sauce is simmering, bring the oil in a large pan over medium-high heat. Add the onion to the pan and cook until it begins to brown, about 5 minutes.
Add the tempeh to the pan, reserving the marinade. Flip the tempeh occasionally so that it browns on both sides. If it sticks, deglaze the pan with the reserved marinade. Cook for about 8 to 10 minutes, or until the onions have caramelized nicely and the tempeh has browned on both sides.
Assemble the Bowls
Divide the noodles amoung 4 bowls and top with the tempeh and onions, and garnish with fresh sage leaves, if using.