In large bowl, combine onions with lemon juice and ginger. Arrange a few onion slices on the bottom of a baking dish(reserve some). Rinse Tilapia, pat dry with paper towel, brush outside with oil, lay fish on top of the onion mixture.
Cut off about 1/2 inch from each of the lemons. Stuff the fish cavity with reserved onions, lemon ends and half of the cilantro. Cut remaining lemons into thin slices; tuck slices around base of fish and sprinkle remaining cilantro over onions and lemons in dish.
Bake, uncovered, in a 400°F oven until fish temperature reaches 135°F and flesh in opaque and flaky. Gently pull off skin and serve fish with onions and a fresh lemon slice. Add salt and pepper to taste.
I think this may take 30-40 minutes*** I have not made this but my gf has :).