Heat vegetable oil in a wok over medium-high heat.
Slice the pork tenderloin into ¼” thick pieces, about 1” in length. Add to a large mixing bowl and toss with cornstarch, salt, and pepper.
Add pork to the wok and fry for 2 minutes per side. Remove and place onto a paper towel-lined sheet or a wire rack-lined baking sheet to allow excess oil to drip away.
Next, add the onions, peppers, and mushrooms to the wok. Season with red pepper flakes. Stir fry for 5 minutes to soften. Remove from the wok and set aside.
Wipe the wok clean and return to the stovetop. Combine garlic paste, ginger paste, soy sauce, brown sugar, and water in a bowl with a pour spout and whisk well. Pour into the wok.
Stire the 1 tablespoon of cornstarch and the 1 tablespoon of water together to make a slurry.
Once the sauce starts to boil, add the cornstarch slurry and whisk. Lower the heat to a simmer. Cook for 3 minutes.
Add the pork and veggies back to the wok and toss in the sauce. Cook for 4 minutes then remove from heat. Garnish with chopped green onions and sesame seeds. Enjoy!