Cut the beef sirloin into thin, 1-inch strips. Massage the pressed garlic and kosher salt into the beef strips, tossing to coat. Melt 1 tablespoon of butter with 1 tablespoon of the oil In a 10-inch skillet over medium-high. Add the beef and stir-fry until browned on all sides, 3-4 minutes. Transfer the sliced beef to a bowl.
To the same skillet, melt the rest of the butter with the oil. Add the onion and peppers and cook for 3-4 minutes. Add the meat back to the skillet and stir in the tomato paste and beef base. Cook for 5 minutes, stirring occasionally.
In a small bowl, whisk the cornstarch, soy sauce, sugar, and water. Add the soy mixture to the skillet and cook for 5-7 minutes, stirring until reduced and thickened. Taste for seasoning and add more pepper if desired.
Serve over cooked rice and sprinkle with chopped green onion.