In a large pot, cook pasta for 8-10 minutes or until desired texture. Drain and set aside.
Break tempeh up into thirds and soak in a bowl of water for 10 minutes. (Do this while pasta is cooking.)
In a large skillet melt butter and saute onion over medium heat until they are transparent in color.
Add tempeh and season salt and cook for 2 mintues or until tempeh is brown.
Next, add vegetable broth, start with 2 tablespoons. Add spinach and stir into the tempeh mixture. Place lid on Skillet/pan and keep stiring occasionally until the spinach is fully reduced.
Reduce heat to simmer and add the sour cream, mustard, garlic powder and nutritional yeast. Mix well.
If the sauce is still too thick, start adding vegetable broth 2 tablespoons at a time until you reach the desired consistency of the stroganoff.
Serve over pasta with a side salad or other choice of vegetable.