Fish Baked in Tahini

From the garden

• 1½ cups sesame seed, unhulled, cleaned of chaff · grinds down to about 1 cup tahini; ready-made tahini works, in which case begin at step 3
• 2 medium yellow onions, halved and thinly sliced
• 4 cloves garlic, minced
• ½ cup flat-leaf parsley, chopped
• ¼ cup slivered almonds or chopped hazelnuts, toasted (optional) · the traditional crunch over the top

From the pantry

• 1½ lb firm white fish fillets, patted very dry
• 2 lemons · juice for the sauce, wedges for the table
• 4 tbsp olive oil · divided between the grinder and the onions
• 1 tsp ground cumin
• 1½ cups long-grain rice · cooked, for serving
• salt · to taste
• black pepper

1. Warm the sesame seed in a dry heavy skillet over medium heat, shaking it constantly, 3–5 minutes, until it smells nutty and the first seeds begin to pop. Tip it straight out onto a cold plate, because sesame goes from perfect to burnt in a matter of seconds and burnt seed cannot be rescued.
2. Grind the cooled seed in a food processor 4–6 minutes, scraping the sides down often. It will go to a stiff sandy paste, then to a clump, and only then loosen into something pourable; add 2 tablespoons of the olive oil once it clumps to help it along. That is tahini.
3. Cook the sliced onions in the remaining 2 tablespoons olive oil in a skillet over medium heat, 12–15 minutes, until soft and sweet and just starting to color at the edges. Stir in the garlic and cumin for 1 minute more, then spread the onions over the base of a baking dish.
4. Whisk the tahini with the juice of both lemons and about 3/4 cup cold water, added a splash at a time. The paste seizes and turns pale and stiff before it gives way, so keep whisking until it flows like thin cream. Season it well with salt.
5. Season the fish with salt and pepper, lay it on the onions, and pour the sauce over so every piece is covered. Bake at 400F for 20–25 minutes, until the sauce has set with brown freckles on top and the fish flakes at the thickest point.
6. Scatter the parsley and the toasted nuts over the dish and serve warm or at room temperature with rice and the lemon wedges.