Preheat the oven to 400°F (200°C).

Heat olive oil in a large skillet over medium heat.

Add thinly sliced onions to the skillet.

Cover the skillet and cook, stirring occasionally, until the onions are very soft and lightly browned (about 15 minutes).

Lightly oil a shallow baking dish.

Place the fish steaks in a single layer in the prepared baking dish.

Sprinkle the fish with salt and pepper.

In a small bowl, combine tahini and lemon juice.

Stir the tahini mixture with a fork until smooth.

Drizzle the tahini mixture evenly over the fish steaks.

Top the fish steaks with the cooked onion mixture.

Bake, uncovered, until the fish is just cooked through (about 20 minutes).

Serve the baked fish steaks with lemon wedges and sprinkle with fresh parsley.