In a medium bowl, combine 1 cup onion slices, bitter orange, tomato sauce, ¼ cup lime juice, ketchup, olive oil, mustard, Worcestershire and Sazón. Season with Adobo and Salsita; set aside.
In medium saucepan over medium-high heat, bring 4 cups water and scallions to a boil. Reduce heat to medium-low and simmer 5 minutes. Add shrimp to saucepan. Cover pan and remove from heat. Let shrimp steep until pink and opaque, about 2 minutes; drain and rinse under cold water. Discard scallions.
Add shrimp to onion mixture. Stir in cilantro. Cover bowl and refrigerate at least 3 hours, or up to 12 hours.
Divide shrimp mixture evenly among serving bowls. Top shrimp evenly with remaining onions and lime juice. Garnish with cilantro, if desired. Serve with popcorn.
Ceviche is an extremely popular way to prepare fresh fish all over South America, Central America and the Caribbean. This cooking technique involves marinating pieces of fresh fish in the juice of citrus fruit (like limes, lemons, oranges and grapefruit). As the fish marinates, the citric acid found in the fruit juice has a chemical effect on the fish’s flesh, altering the structure of its protein. The flesh turns opaque and firm, as if it had been cooked with heat.
When making ceviche, cut fish into 1” pieces, or smaller, to expose the maximum surface area of the fish to the citrus juice, and be sure the fish is submerged in the liquid. Allow the fish to marinate at least 4 hours, or up to 24 hours. You know the fish is “cooked” when it is opaque to its center.
Some fish that work well for ceviche are as follows: firm white fish, like tilapia, sea bass, halibut and snapper; fatty fish, like salmon, mackerel and tuna; shellfish, like shrimp, crab and lobster; and mollusks like conch, clams, mussels, squid and octopus.