Score the plantain skin lengthwise with a knife and separate the peel in sections. Remove the peel and cut each plantain into 1 to 1 1/4-inch thick diagonal pieces (about 8–10 pieces per plantain).

Par-fry

Pour the vegetable oil into a deep skillet or heavy pot to a depth of about 1 to 1 1/2 inches and heat over medium-high until it registers 350°F (175°C) or a small piece of plantain sizzles immediately. Add the plantain pieces in a single layer (work in batches to avoid crowding) and fry for 2–3 minutes per side, until they are just turning golden and slightly softened but not fully browned. Use a slotted spoon to transfer them to a paper towel-lined tray to drain.

Flatten

Combine the 1/2 cup water with 1 tsp salt; this creates a light saline brush to season the tostones. Working one piece at a time, place a par-fried plantain between two pieces of parchment or in a tostonera (plantain press) or on a flat cutting board covered with plastic. Press gently but firmly to flatten to about 1/4-inch thickness. If using the water-salt, lightly brush the cut surface with the salted water after flattening. Optionally, dip the flattened rounds briefly into the cornmeal or breadcrumbs for extra crunch, tapping off excess.

Second fry

Return oil to 350–375°F (175–190°C) if it cooled. Fry the flattened plantains in batches for 1–2 minutes per side, until crisp and golden brown. Remove with a slotted spoon and drain on paper towels. Immediately sprinkle with additional salt to taste (use the remaining kosher salt as needed).

Keep warm

Place finished tostones on a wire rack set over a baking sheet in a 200°F (95°C) oven while you finish remaining batches so they stay crisp.

Guacamole

Prep

Cut the avocados in half, remove the pits, and scoop the flesh into a medium bowl. Finely dice the tomato, mince the red onion and jalapeño (if using), and chop the cilantro.

Mash and season

Using a fork or potato masher, gently mash the avocado to your preferred texture (chunky or smooth). Add the lime juice, minced onion, diced tomato, chopped cilantro, ground cumin, 3/4 tsp kosher salt, and 1/4 tsp black pepper. Stir to combine. Taste and adjust seasoning—add more lime or salt if needed.

Rest (optional)

If not serving immediately, press a piece of plastic wrap directly onto the surface of the guacamole to minimize browning and refrigerate up to 1 hour. Add a final stir and a squeeze of lime if flavor has dulled before serving.

To Serve

Arrange warm tostones on a platter with a bowl of fresh guacamole in the center or on the side. Garnish with extra cilantro leaves and lime wedges. Serve immediately so the tostones remain crisp.