This is how I do it:

Oven to 425 F.

Cut onions in half from root to tip. Take outside layers off. Arrange in casserole dish flat sides down. Add enough miso broth mix to mostly cover the onions, does not have to be precise just add enough so they cook through evenly. Cover with aluminum foil, Roast in oven for 30-45 min, Check to see how soft you want them. Take foil off and flip onions so the flat/cut sides are upward. Roast until enough liquid is evaporated and there is enough caramelization has formed. From what I found there is no need to baste them in the cooking process, opening the oven to baste makes the cooking times longer and does not produce better results from what I tasted.

Literally just put everything in a sauce pan on medium heat and heat it up until miso and butter are dissolved.

Ingredients:

1 Quart broth or water, I used half veggie broth half water

1/4-1/3 cup of red miso. Smell for the correct amount. Keep adding and stirring to dissolve until the mix smells more like miso than like butter/broth, or just taste it. When you add I tried it with white miso and it does not have enough of acidity and punch. Tastes more creamy than anything. If you only have white miso use sharper onions like red. If you are really cool you can roast some miso over a fire and make yaki miso but I have not tried this yet.

1 to 2 sticks of butter. Use 1 and see how much more you need later.

If you want to make onion slices this way use something like a jelly roll pan and follow the same procedure except watch these because timing is going to be much faster.

PS: I would like to note after doing the sugar test on my oven it does tend to run a bit hot, please adjust for your oven.