ginger
small red onion
red Thai chiles
fresh or bottled calamansi juice
coconut or cane vinegar (such as datu puti), divided
sashimi-grade yellow fin tuna or Spanish mackerel, or raw shrimp
kosher salt, plus more as needed
freshly ground black pepper, plus more as needed
full-fat unsweetened canned coconut milk (optional)
Chef's knife and cutting board
Mixing bowls
Fine-mesh strainer
Large spoon