Sprinkle halibut with seafood seasoning; place into a container with a lid.
Whisk balsamic vinegar, lemon juice and zest, parsley, thyme, and rosemary together; pour over halibut. Cover; marinate fish in the refrigerator for 2 to 4 hours.
Preheat oven to 350 degrees F (175 degrees C). Grease a baking dish; scatter onion evenly in the bottom of the dish.
Heat oil in a skillet over high heat; fry halibut just until browned, about 1 minute per side. Remove halibut and place on bed of onions in prepared baking dish.
Bake in the preheated oven for 8 minutes. Remove fish; pour about half the marinade over the fish, discarding remaining marinade. Return halibut to the oven; bake until fish flakes easily with a fork, about 7 minutes more.