Halve steak lengthwise; freeze for 15 minutes to firm slightly. Thinly slice crosswise.
In a small bowl, combine tangerine juice, oyster sauce, soy sauce, sherry, ginger, garlic, cornstarch, sriracha and a generous grind of pepper.
Warm a 12-inch skillet over medium-high heat until very hot. Add 2 tsp. oil; swirl to coat. Add half of steak; sear for 1 minute. Toss until steak just loses its raw color, 30 seconds longer. Transfer to a plate. Repeat with 2 tsp. oil and remaining steak. Pour off and discard any liquid in skillet.
Add remaining 2 tsp. oil to skillet; swirl to coat. Add snow peas and stir-fry for 1 minute. Add scallions; stir-fry for 30 seconds. Add sauce, reduce heat to medium and simmer for 1 minute. Return steak to skillet and cook until just warmed through, about 1 minute. Stir in zest. Serve with rice, if desired.