In a small bowl, whisk together the cornstarch and water to create a slurry; set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat until shimmering.
Add the chicken cubes in a single layer and cook for 6 to 8 minutes, turning occasionally, until golden brown and cooked through to an internal temperature of 165F.
Remove the chicken from the skillet and set aside on a plate.
In the same skillet, add the sliced onion and jalapenos; sauté for 3 to 4 minutes until the vegetables are softened and slightly charred.
Add the minced garlic and cook for 30 to 60 seconds until fragrant.
Return the cooked chicken to the skillet and stir to combine with the vegetables.
Pour in the soy sauce, oyster sauce, and sesame oil, tossing to coat all ingredients.
Stir the cornstarch slurry once more and pour it into the skillet; cook for 1 to 2 minutes until the sauce has thickened and glossed.
Season with black pepper and serve immediately over steamed rice.