Prep and cook instructions are identical for both fish options.

• Trim the root end from the leek; cut the leek in half lengthwise, then crosswise into ¼-inch-thick half-moons, enough to measure 1½ cups [3 cups]. Soak the half-moons in a medium bowl of cold water, stirring occasionally to loosen any dirt, and allow the dirt to settle at the bottom. Skim the leeks from the surface of the water and pat dry.
• Finely chop, press, or grate enough garlic to measure ½ teaspoon [1 tsp].
• Slice the olives crosswise, checking for any pits.
• Cut the tomatoes in half.
• Zest the lemon. Juice half; cut half into wedges for garnish. [Zest both lemons. Juice 1 lemon; cut 1 lemon into wedges.]
• Strip the parsley leaves from the stems; coarsely chop the leaves. Set aside half for garnish.
• Pat the fish dry with a paper towel; season generously with salt and pepper.

In a large frying pan over medium heat, warm 1 to 2 tablespoons oil until hot but not smoking. Add the leek and garlic, season with salt and pepper, and cook, stirring occasionally, until the leek is softened, 3 to 4 minutes. Add the olives, tomatoes, lemon zest, half the parsley, 1 tablespoon [2 TBL] lemon juice, ¾ cup [1½ cups] water, and as many chile flakes as you like. Bring to a boil, then reduce to a simmer. Nestle the fish in the sauce and cook until the fish is opaque and flaky, 6 to 8 minutes. Remove from the heat and season to taste with salt and pepper. While the fish is cooking, prepare the smashed white beans.

• Rinse the white beans.

In a medium bowl, using the bottom of a bowl or cup, mash the beans to a coarse paste. Season to taste with salt and pepper.

Transfer the smashed white beans to individual bowls. Top with the fish, vegetables, and sauce. Garnish with the remaining parsley and serve with the lemon wedges.