For the pickled red onions: In a medium saucepan over high heat, add the vinegar, sugar, peppercorns and 2 tablespoons salt. Mix well and add the beets and onions. Once it comes to a boil, stir to dissolve the sugar and salt then remove from the heat. Allow to cool then cover and refrigerate at least 1 hour and up to overnight.
For the grouper: In a small bowl, add the orange zest, juice, olive oil, ground achiote and cumin. Mix well to combine, then pour the marinade into a large resealable plastic bag. Add the grouper filets and seal the bag tightly, squeezing out any excess air from the bag. Gently mix around so the marinade coats the fish well. Refrigerate for 45 minutes.
Clean the grill grates and preheat a grill to medium-high heat.
Arrange 2 banana leaves to form a cross. Remove one piece of grouper from the marinade and place in the center of the leaves. Season with 4 to 5 turns freshly ground black pepper and 1 teaspoon salt. Wrap the fish up tightly in the banana leaves by folding over one flap at a time, alternating each side so they overlap. Secure the last folds of the leaves together with toothpicks to hold in place. Place the banana leaf-wrapped grouper on the grill, and cook until the leaves turn bright green and begin to char around the edges, 5 to 6 minutes per side. Remove from the grill and set aside.
To serve, cut open the banana leaves to expose the fish, add a large spoonful of Spicy Grilled Corn Salsa and garnish with the pickled red onions, chopped scallions and cilantro.
Spicy Grilled Corn Salsa:
Preheat a grill to medium-high heat.
Place the corn on the grill and cook until well charred all over, 5 to 6 minutes per side. Drizzle the poblanos and fresnos with olive oil and season with 5 to 6 turns freshly ground black pepper and 1 teaspoon salt. Place the chilies on the grill and cook until lightly charred on all sides, 3 to 4 minutes. Place the chiles into a bowl, cover tightly with plastic wrap and allow to steam for 3 to 4 minutes.
Hold a corn cob upright in a large mixing bowl. Using a chef's knife, cut off the kernels and let them fall into the bowl. Repeat with the remaining corn. Set aside the kernels. Discard the cobs.
Remove the skins from the chiles. Mince the fresnos, dice the poblanos and add them to the bowl with the corn kernels. Add the red onions, scallions, lime zest, juice, garlic, agave and 2 tablespoons olive oil. Stir to combine and season with salt and pepper. Refrigerate until ready to serve.