To assemble the nachos: Place a single layer of plantain chips on the bottom of an oven safe tray. Sprinkle some of the cheddar and Monterey jack cheese on top. Place diced onion and jalapeuño slices on top of the cheese. Repeat with the remaining plantain chips, cheese, jalapeuños and onions to create 3 more layers, making each layer slightly narrower than the previous one.