Prep the mushrooms, you can either slice them or dice them. Slices will give you less mushrooms per bite, but bigger pieces, whereas diced will give you more little pieces of mushroom per bite. If you plan on straining the gravy to be smooth then it doesn't really matter which one you go with! I only used 4 oz (114g) of mushrooms, but feel free to add more if you like. You may need to adjust the liquid levels though!
Add the mushrooms to a cold, dry pan and set the heat to medium. Cook them for about 6 - 8 minutes or until they have released all their water. Stir occasionally.
Add in 1 Tbsp of the vegan butter and stir to melt. Then add in the onions and cook for about 5 minutes or until softened. Add in the garlic and stir and cook for one minute.
Next add in the other 2 Tbsp of vegan butter and stir to melt. Stir in 1 - 2 Tbsp of flour at a time, until you see no more dry flour. Cook, stirring frequently for 2 - 3 minutes.
Then add in the Broth about 1/2 cup at a time, and stir / whisk to incorporate. Repeat with enough broth until the gravy is at a consistency that you like. Keep in mind it will thicken as it cools as well! You can always add more broth, but you'll need to reduce it to thicken if it's too thin for you.
Stir in Dijon Mustard, Vegan Worcestershire and Gravy Master if using. Taste and adjust for seasoning. If gravy feels too thick, add in a splash of broth until it's at your desired consistency. Or if it's too thin, simmer and reduce.
You can add a sprig of rosemary and thyme and simmer for a few minutes if you want to infuse with some herbal flavors.
The gravy can be left chunky with the veggies, or you can strain through a fine mesh strainer to make a smooth gravy. Whichever texture you like will work!
Enjoy immediately on mashed potatoes!
Store in the fridge for 3 - 5 days. Gently reheat on the stovetop. Or microwave in 30 second increments and stir until heated through.