Add vegan butter to a deep skillet or cast iron pan over high heat. When the butter has completely melted, add chopped mushrooms and rosemary. Cook for about 4 to 5 minutes, until the mushrooms have significantly shrunk down and released their juices.
Reduce heat to medium-low and add flour. Stir until the mushrooms are evenly coated. Cook for about 1 minute, until the flour begins to turn into a paste. Then, slowly pour in non-dairy milk, starting with only 1 ½ cup, stirring constantly with a wooden spoon or a whisk to avoid too many lumps. Season with salt and pepper, to taste, and continue cooking for about 5 more minutes, until the liquid has reduced and the gravy is thickened to the right consistency. If you enjoy your gravy on the thinner side, continue adding a little more milk, 2 tablespoons at a time, until you reach your desired consistency.
