Dry-fry onions and mushrooms over medium heat until brown ~3-5 minutes. Deglaze pan by pouring in mixed bouillon stock.
Add pasta, nutritional yeast, black pepper, white pepper, garlic. Bring to a boil over high heat, then reduce temperature to medium-low, and let simmer for 10-15 minutes, stirring occasionally, making sure nothing is sticking to the bottom of the pan.
Turn off the heat, stir in vegan butter, lemon juice, and salt to taste.