In a non-reactive bowl or the Instant Pot inner pot, combine the steak strips, sliced onion, and the lime and lemon juice. Add the chili powder, brown sugar, garlic powder, oregano, cumin, 3/4 teaspoon of salt, 1/2 teaspoon of black pepper, and the beef stock. If desired, add about 1/2 teaspoon of crushed red pepper flakes. Mix well and let stand at room temperature for 15 to 20 minutes or cover and refrigerate for up to 2 hours.