Fish in Tahini Sauce W/Roast Lemons

I only use my tahini for hummos, which I don't make that often, so I was pleased to see another recipe I could use it in. My trip to Central Market for veg isn't til next week, so I'm having to forego the onions in this one, at least this time around.

white fish fillets
salt, olive oil
3 lemons
1 cup tahini (sesame seed paste)
1 cup water
2 large onions, thinly sliced
cilantro for serving

Preheat the oven to 200C. Rub fish with salt. Juice 2 of the lemons and slice the 3rd lemon. Combine tahini with the lemon juice and stir until the mixture is thick and lightened in color. Stir in the water to form a thin sauce. Lightly oil a baking dish and place the lemon slices in it. Place the fish over the lemon slices and rub with a little oil. Place the fish in the oven and roast for 20 minutes. Meanwhile, saute onions in some oil until softened. Add the onions to the tahini sauce. After the twenty minutes, remove the fish from the oven, it should be thick and opaque. Pour the tahini sauce over the fish, scattering the onions on top (optional: reserve a little extra tahini to drizzle on before serving). Return the fish to the oven and bake another 30-35 minutes, or until the sauce has thickened and is bubbling all over. Let cool before serving, with cilantro as garnish.