Budge

"...certain undesirable volatiles that would remain behind in covered stock." I think you've hit on something here. Julia Child, in her very first book, Mastering The Art of French Cooking, in her section on making stocks, said (paraphrasing) you would think a pressure cooker would be an ideal means of making stock, but the result is not good. I'm thinking there would be some "yuk" left behind from pressure cooking beef and particularly lamb, perhaps less of that with pressure cooking chicken and perhaps none of that with pressure cooking vegetables for stock.

Personal taste preference is all I can figure. I love the taste of mushrooms, and while adding them to fish stock is a bit unusual, the taste combination might be good. I like the taste of Dashi, that combination of dried Kombu seaweed and dried Bonito flakes used in making Miso soup, and it's quick to make, so I use Dashi whenever a recipe calls for fish stock. Guess that borders on the unusual/bizarre!

So melt them first by steaming them with the cover on, then cover off, heat up, stirring constantly to brown. Makes sense.

I use a very small amount of oil and butter. Low heat will cause them to stew, you need the water released to evapoate. You're actually cooking the sugars not the onions to get the lovely dark caramel. ← Actually cooking the sugars, not the onions, to get the dark color makes perfect sense to me. I'm thinking of almost dry pure sugar in a pan over heat when it turns to pure caramel with that amber color.

"or you get either too hungry or too bored!" You hit it right on.

My question is absurdely simple, yet I can't tell you how many times I've impatiently burned onions by turning up the flame when trying to brown a big batch of onions in a large Le Cruset pot in preparation for onion soup, or even just trying to get a few brown strands in a smaller pan to go atop a hamburger. Love that sweet caramely taste, but I get restless standing over a pot of onions for an hour! Can I get away with turning the flame down to very low and just leaving it and coming back and stirring it every 15 minutes, say? It'd probably take two hours that way, but I could live with that. Do you brown them in all butter, or half butter half vegetable oil? Maybe add a little water every so often to lessen the likelihood of them burning? Guess there's no easy way to speed it up.