Preheat your oven to 425F (220C). Lightly grease a pizza pan.
For the mushrooms: melt the butter in a large skillet over medium-high heat then add in the onions and garlic. Sauté for a couple of minutes until they begin to soften. Add in the mushrooms and satué until they have shrunk in size and cooked through, about 5 minutes. If needed you can add a small splash of water to help the mushrooms from sticking to the pan. Remove the mushrooms and onions from the pan and set aside. Return the pan to the heat.
For the coconut sauce: Add more vegan butter to the pan and when melted sprinkle in the flour. Stir the flour into a paste and cook for 1 minute. This makes a roux which will help thicken the sauce. Whisk in the coconut milk along with all the remaining spices and seasonings. Cook the sauce while whisking for about 5 minutes until it thickens. It should leave a thick coat on the back of the spoon. Remove from heat and let cool a bit.
To prepare the pizza: On a lightly floured surface, stretch the pizza dough until it fits the pizza pan. Spread the mushrooms over the pizza dough, and then pour the slightly cooled coconut sauce over top. Bake for 18 to 20 minutes until the crust is golden brown and the coconut sauce is bubbly and lightly browned. Let cool 10 minutes before slicing.