In small saucepan, combine the vinegar, sugar, and allspice and simmer until the sugar dissolves. Set aside.
Heat the oil in a medium-size skillet over medium heat; stir in the onion and cook until translucent, about 4 minutes. Add the serranos, allspice-vinegar syrup, and salt and gently simmer until the mixture binds together, 8 to 10 minutes.