Slice the chicken breasts diagonally into strips about 1/3-inch (1 cm) wide and place them in a bowl; season lightly with salt.
Heat 2 tablespoons vegetable oil in a wok or large skillet over medium-high heat until shimmering.
Add the sliced onions and stir-fry for about 3 minutes, until they begin to soften and wilt.
Add the seasoned chicken strips and the sliced chiles to the pan, then add 1/4 cup (60 ml) Thai chile jam (or 2 tablespoons chile paste), 1 tablespoon oyster sauce, and 1 teaspoon fish sauce.
Stir-fry everything together, tossing frequently, until the chicken is cooked through and the sauce coats the ingredients, about 5 more minutes.
Taste and adjust seasoning with a little more salt if needed, then remove from heat and serve immediately.