Making the escabeche. In a very large (12-inch) skillet, heat the olive oil over medium. Add the chile pieces and stir until fragrant, about 1 minute. (Don’t cook them too long, they’ll burn and taste bitter.) Scoop in the red onion and cook another minute while the onion wilts slightly. Add the balsamic vinegar, sugar, 1/3 cup water and 3/4 teaspoon of salt. Simmer another minute to allow the flavors to come together.
Finishing the shrimp. Add the shrimp to the skillet with the escabeche. Stir until they are just barely cooked through (they will turn pink but be slightly translucent at the center), 6 to 8 minutes. Scoop the shrimp into a shallow serving bowl and garnish with the cilantro leaves. This dish is delicious served warm, at room temperature or cool.