Trim the tops and bottoms from the fennel bulbs, reserving a handful of the fronds for garnish. Quarter the bulbs. Set each quarter on its side and slice off most of the core. Cut the quarters into wedges about 1-inch thick (don't worry if some pieces fall off the wedge-use those too).
Put 1/4 cup oil in a large skillet with a tight-fitting lid. Place over medium-high heat until almost smoking. Add the fennel and cook without stirring until fennel is brown on the bottom, about 5 minutes. Stir and let the fennel continue to cook until some more pieces brown, 1 to 2 minutes.
Add 1/2 cup water to the skillet, scraping up any browned bits from the bottom. Sprinkle with 1/2 teaspoon of salt and several grinds of pepper. Lower the heat to medium, cover the skillet, and cook 5 minutes.
Sprinkle the cod with salt and pepper and place on top of the fennel. Cover the pan again and simmer, adding more liquid occasionally if the pan is dry, until the fennel is very tender and the cod is cooked through, 5 to 7 minutes.
Meanwhile, chop the reserved fennel fronds (you should have about 1 1/2 to 2 tablespoons) and remove the zest from the lemon. Heat the remaining 2 tablespoons of oil in a medium skillet over medium-high heat. Add the breadcrumbs and cook, stirring frequently, until the breadcrumbs brown, 2 to 3 minutes. Remove from the heat, season with salt and pepper, and allow to cool. Mix the fennel fronds and lemon zest into the breadcrumbs until combined.
Juice the lemon. Roughly chop the olives.
When the cod and fennel are done, stir in the lemon juice and the olives. Remove from the heat and let stand for about 1 minute to warm the olives. Transfer the fennel and cod to serving bowls and top the fish with the toasted breadcrumbs. Pour any remaining pan juices over the fennel.
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