Cook the pasta according to its package instructions. Then, drain and rinse it off under cold water to stop it from cooking. Drizzle a touch of olive oil over the pasta and toss it to coat it evenly; this will prevent the pasta from sticking to itself. Set aside.
Place a large saute pan over medium heat. Once hot, add in the olive oil and butter.
Once the butter is melted, add in the diced onions and saute them for 2 minutes, stirring occasionally.
Add all of the mushrooms into the pan, stir well, and cook for 6 minutes, stirring occasionally.
In the meantime, whisk together the hot water and bouillon cube. This is your vegan beef stock. Set it to the side.
Add the flour, garlic powder, onion powder, salt, and pepper into the pan and constantly stir for 1 minute.
Now, slowly pour in the vegan beef stock, constantly stirring to create a sauce.
Once all of the stock has been stirred in, add the sour cream and dijon mustard into the pan and stir until well incorporated.
Continue cooking over medium heat and allow the mixture to come to a simmer. Reduce the heat to medium-low and simmer for 3 minutes.
Add the cooked pasta into the pan and stir well to coat it evenly. Cook for another 1 to 2 minutes, or until desired sauce thickness.