Bistek (Bistek Tagalog) is a Filipino dish made from marinating thinly pounded pieces of sirloin in soy sauce and calamansi juice. It is covered in onions, occasionally pan-fried potatoes, and served with rice.
The Calamansi (Kalamansi, Calamondin Orange, Golden Lime, Chinese Orange) is a type of citrus native to Southeast Asia. It is a cross between the Mandarin Orange and Kumquat. The fruit has an orange sour flesh and a thin, sweet rind that starts green, but turns orange as it ripens. The rind is often eaten with the flesh to produce a sweet sour flavor. It is commonly found in Filipino cooking, from garnishes to condiments. I was unable to find calamansi (my mother-in-law used to have a tree when she lived nearby), so I used the juice of a Meyer Lemon.The beef still tasted great, but just not quite the same flavor as calamansi.
More information on Calamansi.