Combine pork, 1 teaspoon cornstarch, salt, and pepper in a small bowl, tossing to coat.
Combine water, oyster sauce, sugar, Sriracha, and remaining 1 teaspoon cornstarch in a small bowl.
Heat oil in a large nonstick skillet over medium-high heat. Add minced garlic and pork mixture; sauté 3 minutes or until the pork is done. Add chopped tomatoes; sauté 1 minute. Add cornstarch mixture; cook 1 minute or until thickened. Add basil; stir to combine. Remove from heat, and sprinkle with onions. Serve with brown rice.