Wash and dry ½ bunch cilantro. Coarsely chop leaves and stems—you should have about 1 loosely packed cup. Trim roots and top inch of green parts of 4 scallions; discard. Cut scallions into 1″ pieces. Cut ½ habanero chile in half lengthwise and remove seeds from one half (or leave them in, if you don’t mind extra heat).
Transfer cilantro, scallions, and chile to a blender. Squeeze in juice from 2 limes; add 2 Tbsp. water. Season with a big pinch of salt. Blend on medium speed until cilantro and scallions are finely chopped, about 15 seconds.
Add 1 cup sour cream or Greek yogurt and blend on high speed, scraping down sides as needed, until sauce is pale green and frothy, about 1 minute. Pour into a serving bowl and chill while you fry the plantains (or cover and chill sauce up to 1 day).
Pour vegetable oil into a large (10″) cast-iron skillet, Dutch oven, or other large heavy pot to a depth of ½”. Heat oil over medium-high until an instant-read thermometer registers 375°, 7–8 minutes.
Meanwhile, prep the plantains: Trim off both ends of the plantains. Using the tip of your knife, score skin of a plantain lengthwise down its ribs, rotating and repeating until you’ve scored the plantain all the way around. Make sure your cut extends down to the flesh of the plantain, but try not to cut into the flesh itself. Peel plantain along your score marks, then cut crosswise into ½”-thick coins.
Line a rimmed baking sheet with a wire rack, or with a double layer of earth-friendly paper towels. Working in 2 batches, fry plantains, turning occasionally to ensure they don’t stick to each other or to the pan, until golden and starting to crisp, about 2 minutes.
Using a fish spatula or slotted spoon, transfer plantains to prepared baking sheet. Repeat with remaining plantains.
Reduce heat to low to keep oil warm while you flatten the plantains. Using a meat mallet or a flat-bottomed dry measuring cup and working one by one, place plantain on cutting board and firmly press down on plantains to flatten them to less than ¼” thick; the edges should split and become craggy. (Do this while the plantains are still warm; otherwise, they become annoyingly difficult to flatten.)
Reheat oil to 375°. Working in 3 batches, fry flattened plantains, turning occasionally and letting oil return to 375° between batches, until dark golden brown and crisp, about 3 minutes. Transfer to prepared baking sheet; season immediately with salt (it sticks better when the tostones are hot!).
Transfer plantains to a platter. Serve with green sauce alongside for dipping.
Do Ahead: Plantains can be fried 30 minutes ahead. Transfer to a wire rack set in a rimmed baking sheet and place in a 250° oven to keep warm.