They taste just like a starchy root vegetable with their own flavor of course, what goes good with them.
Feed them to the pigs! Gah! I hated guenios when I was young and my 'uela made me eat them. O.K., with that said, here is one way she forced me to eat them:
Ensalada Bacalao. That salad has green bananas AND yucca! There’s a reason there’s a ‘yuck’ in ‘yucca’.
One way she didn’t have to force me to eat them was in pasteles Mmmmm, pasteles. We only get them on holidays because they are a major pain in the ass to make. But man, oh, man that’s some good eatin’!
Are you talking about the bananas people usually eat raw, as in peel it and eat it, and it tastes sweet? Or are you talking about boiled plantains, the much starchier cousin that is hard to eat without cooking first and use it in various recipes?
If it is the first one, and has already been boiled, then chop it up and add some garlic or onion or both and drizzle with olive oil. Ensaladas de bacalao is delicious, yo! 
If it is the former, smash them, mix with garlic and olive oil, and you have the basis of mofongo, basically mashed plantains (instead of mashed potatoes). Good side dish, delicious. You can even add bacon to the mash plantains and mix!
Sweet bananas picked while still green before turning yellow and boiled.
The ensalada de bacalao (cod salad) can be used with them, then. Or drizzle with olive oil and sauteed onions and garlic… Simple, but good.
Mmmm… Guineos verdes en escabeche here is another recipe… Too many things for my taste, but you may want to try.
Seconded. The version drizzled in olive oil with garlic as well as the escabeche are good as a side to slow-roast pig with pepper/garlic rub.
Damn you, people, I’m trying to stick to the diet until the holidays roll around! 
Ew, WHY would you boil them. That sounds bland to the nth degree. Is it too late to squash them and make some form of tostones?
I just got a veggie shredder… You just gave me the idea for the next project… arañitas! 
Boiling, smashing, and adding oil, garlic, and bacon are the basis for mofongos (mentioned above).
Tostones are smashed individually, then fried as little patties. It’s just that usually they are twice fried, not boiled and fried. I wonder if it would work…
I know, which is why I suggested mofongos instead of tostones. I was told by a dietician, though, that if I wanted less fried tostones I could microwave the tostones first and then fried them… I have not tried that, nor the boiling and frying way. Double fry is easier.
Correct me if I’m wrong but aren’t tostones made from something different than guineo? Plantains are delicious, green bananas-- not so much.
Also tostones are fine, fine things but I loves me some fried amarillos. Or maduros. Or whatever you call sweet plantains.
Yep, tostones are traditionally made with plantains, not bananas. That said, I’ve tried * tostones de pana* (breadfruit), so perhaps the starchy green bananas could be used fried… It’d be a good experiment, at least.