Meriad the first few casseroles I did weren't very impressive, then I realised they'd been cooking for too long.
Now as you know, some of us pre-fry the meat, onions, etc, and some don't.
I do, because it's how I would do a casserole in the oven as well, and I believe you get extra flavour from that initial frying, so that's the method I'll describe
First put a tablespoon or so of plain flour in a bowl or plastic bag, and add salt & pepper plus if you like a teaspoon of mixed herbs, or a small spoonful of English mustard powder, or a teaspoon of ground ginger. Adding them now gets the flavour straight into the meat at the beginning.
Heat some oil to fairly hot, then start browning the meat in batches; don't add too much at once or it will sort of boil instead. Let it sit on the first side for a good few minutes before turning; it should be nicely brown, even a bit crusty. Have your s/cooker pot ready and transfer the pieces to that as they're browned.
Turn the heat down a little, and fry some onion, carrots and celery if you have it. I tend to thinly slice or chop some onion and some leave in bigger wedges. Carrots in fairly thick slices, celery thinner. Garlic can be added now too, but not too early or it will burn.
Stir them well, scraping up all the lovely brown bits in the pan. Once browned a bit, pour in some red wine, or beer, stir it in really well, then add 1 or 2 beef oxo cubes and boiling water. I can't say how much, but when it goes into the s/cooker, it wants to
just cover the meat etc. A bit of tomato puree is good, and/or some Dijon or wholegrain mustard (depending on what you've added to the meat).
Add the meat back in, then as soon as it's boiling tip it all into the s/cooker. If you do this, you don't need the cooker on high at all, and even for shin of beef the most I've found it needs is 7 hours on low, but I usually allow for about 6.
Taste it after about 3 or 4 hours, before that you won't get the proper flavour.
If you want mushrooms in it they can go in whole, or halved, at the same time as the liquid. Or, actually nicer, is to later briefly sauté whole shallots and mushrooms, then add them about 2 hours before the end.
Hope that helps
And hopefully I'll be here on Friday for a live cookalong
If you don't want to do the pre-frying etc then I would start it on high for probably the first hour but no more.