Step 1

Set a fine-mesh sieve over a small bowl. Purée first 4 ingredients and 4 large ice cubes in a blender until smooth. Add onion; pulse 3–4 times. Strain liquid into a medium bowl. Stir in clam juice, if desired; season with salt. Cover and chill.

Step 2

Pour water into a large pot fitted with a steamer basket to a depth of 1 inch; bring to a boil. Add sweet potato, cover, and cook until just fork-tender, about 30 minutes. Transfer to a plate; let cool.

Step 3

Meanwhile, add more water to same pot, if needed, to measure 1 inch; bring to a boil. Add ear of corn to pot and steam until crisp-tender, 2–3 minutes. Transfer to a plate; let cool completely.

Step 4

Halve potato lengthwise. Using a small melon baller, scoop out potato balls and place them in a small bowl; set aside. Cut kernels from cob. Reserve 1/3 cup kernels (save extra kernels for another use).

Step 5

Rub a large bowl with cut sides of chile; discard. Place fluke, 2/3 of onion, leche de tigre, and 4 large ice cubes in bowl; stir well. Let marinate for 2 minutes; remove ice. Fold in potato and corn; season with salt.

Step 6

Using a slotted spoon, divide ceviche into small bowls or onto plates. Drizzle ceviche with leche de tigre from bowl; garnish with remaining onion and cilantro.