What you’ll need:
Rinse the fish in cold water, then dry with paper towels.
Set a steamer basket over a saucepan of simmering water, making sure the bottom of the basket doesn’t touch the water. Add sweet potatoes and steam for 7 minutes, or until tender. Run the sweet potatoes under water, then drain and set aside.
Combine the red onion, chilies,garlic, lime juice, lemon juice, salt, cilantro, and culantro and mix well. Add the fish and gently toss to incorporate. Let marinate for 10 minutes.
Add the sweet potatoes and raw corn and toss again. Taste and season with more salt if needed and some pepper. Let chill at least 1 hour in the fridge, then serve.
Note: Fish for ceviche should always be fresh, not frozen