I don't really do recipes and made this up myself - it's more technique than recipe...
Oven at 350
Veg quantity is roughly 1/3, 1/3, 1/3, but you can have more or less of whatever is your favorite (my fam LOVES mushrooms and onions, so we really increase those, but you can also do 1/2 green bean and 1/4 of each of the others if you like beans more - and I tend to use 2 types of mushrooms)
Prep the green bean (snap the ends) - toss on the parchment lined pan
Prep the mushrooms by cleaning and slicing (dice ends and thinly slice caps) - toss on the pan
Prep the onions with a dice (about size of mushroom or a little smaller - both much smaller than green bean) - toss on the pan
Drizzle with olive oil (your amount choice) and s&p and hand toss
Bake at 350 for 30 minutes - see if you like your veg cook (some folks like veg more or less cooked and some folks have more or less veg on the pan)
If you do, add a handful of fried onions on top (your choice on amount) - bake 5-10 more minutes, depending how crispy you like your topping
Serve
It's great - my kids ask for this as the main veg every year now. Everyone always wants the crisp onion topping on veg and this delivers - no sog on the fried onions...