Step #2 Heat 1/2 tbsp oil in a 10- to 12-inch nonstick frying pan over medium-high heat; when hot, turning once, until browned on both sides & opaque but still moist-looking in the center (cut to test), add fish & cook, 8 to 10 mins total.
Step #3 Transfer to a shallow 2- to 3-quart glass or ceramic dish.
Step #4 In the same pan, heat remaining tbsp oil over medium-high heat; add onion & stir until limp & browned, about 8 mins.
Step #5 Spoon onion over fish.
Step #6 In pan, mix vinegar, peppercorns, bay leaf, sugar, allspice, garlic, salt, & 1 1/4 c water; bring to a boil over high heat.
Step #7 Remove from heat & pour over fish & onion.
Step #8 Let stand until cooled to about room temp, about 15 mins, then cover & chill at least 8 hrs or up to 1 day.
Step #9 Let come to about room temp.
Step #10 To serve, lift fish from liquid & arrange on a platter.