Sometimes known as calamondin in the United Sates, calamansi is a sweet-tart citrus that’s a favorite with Filipino cooks. Fresh calamansi can be difficult to find, so we recommend purchasing 100% pure calamansi juice with no sugar added, which can usually be found pasteurized in bottles or packets or in the freezer section. Alternatively, substitute lime or lemon juice. If you have one, use a mandoline to slice the onion thinly and evenly, otherwise a sharp heavy knife can do the job as well. The bistek tagalong can be refrigerated in an airtight container for up to 4 days.