Start by finely dicing the red onions and placing them in a small bowl. Pour ¼ cup of Datu Puti vinegar over the onions and let them sit while you prepare the mushrooms, about 20 minutes until they turn pink. This mellows the raw onion flavor while adding a nice tangy brightness.

Preheat your oven to 375°F (190°C). Wash and thoroughly dry your mushrooms – this is important for proper roasting. Arrange the mushrooms in a single layer on a baking sheet, making sure not to overcrowd them. Use two baking sheets if needed. Drizzle approximately 2-3 tablespoons of olive oil over the mushrooms and season generously with salt and pepper.

Roast the mushrooms for 10 minutes, then take them out and flip them over. Continue roasting for another 10 minutes until they're golden brown and slightly crispy at the edges. Remove from the oven and let them cool to room temperature. Once cooled, chop the mushrooms into small, bite-sized pieces and transfer them to a large mixing bowl.

To the bowl of chopped roasted mushrooms, add 4 tablespoons of the pickled onions including some of the vinegar, 2 tablespoons coconut aminos, 2 tablespoons calamansi juice, 1 tablespoon vegan mayonnaise, and 1 tablespoon of chopped chives or green onion. If you want some heat, add your preferred amount of red pepper flakes, hot sauce, or fresh chilies. Mix everything thoroughly until all flavors are well combined. Taste and adjust the seasoning with salt, pepper, additional vinegar, or calamansi juice as needed. For best flavor, let the mixture rest for 5-10 minutes before serving.

For cold serving (like a salad), simply transfer the mixed sisig to a serving platter. Garnish with additional pickled onions and chopped chives or green onions. Serve with extra hot sauce or chili on the side for those who like more heat.