Whisk all the sauce ingredients together in a small mixing bowl. Set aside until ready for use.
Chop and prepare all ingredients as listed. For the textured veggie shreds, prepare as instructed. veggie shreds would normally come dehydrated so you will most likely need to rehydrate in hot water. Drain and press out excess liquids after rehydrating. You want to remove as much liquids as possible. For the mushrooms, chop into small bite sized pieces or into small squares. This will also help king oyster mushrooms cook faster as they are slightly firmer in texture. Preheat any sauté pan or cast iron skillet to medium high heat. Drop one tablespoon of vegan butter onto the pan to melt. Once your skillet is fully heated, add in the king oyster mushrooms and sauté into the butter. Sprinkle with a pinch of salt & pepper. Leave alone for 1-2 minutes to caramelize in color and shrivel up in size. Toss around once more and leave for another minute or until it is shimmery and slightly golden brown in color. Remove from the pan or set aside. Toss the remaining tablespoon of vegan butter. Add in the aromatic ingredients such as the red onions, garlic, ginger, and bird's eye peppers. Sauté until fragrant and onions are transluscent. Mix the mushrooms back in and stir. Then add the veggie shreds and mix.
Gradually pour sauce, about a tablespoon at a time and stir into sisig mixture until ingredients caramelizes in color. Season a little salt and pepper to taste. Turn off the heat and splash freshly squeezed calamansi juice over the sisig.
Add green onions for garnish and serve with vegan mayonnaise as a condiment. Best eaten when freshly cooked and over a bed of rice OR with an ice cold beverage - preferrably carbonated for best pairing.