First, thinly slice the fillets and then cut them into 3/4-inch bite-sized squares. Keep them very cold while you prepare the rest of the recipe.
Combine the ingredients for the marinade in a bowl. Set that bowl inside a larger one, filled with ice. Now prepare the salad.
In a small bowl, whisk together the olive oil, onion, cilantro, cumin, and salt and pepper. Then, in a larger chilled bowl, toss the vinaigrette with the avocado and jicama. Set aside.
Gently mix the grouper and the marinade/dressing and let stand for about 10 minutes. Into 4 chilled bowls, spoon the ceviche with its liquid. Serve the salad, family-style, and encourage the guests to top their ceviche with salad.
If you can get Cancha (Latin American corn nuts) and toast, they make a fantastic and authentic addition to this dish. I sometimes like to drizzle coconut milk onto the citrus juices. Or, I season the dish with tiny dots of soy sauce and sesame oil.