Yes, do brown the meat a bit. If only because it looks better when it comes out. Does the recipe say onions as well as onion soup ? That sounds odd. I'd prefer onions ( fresh, just wearied a bit in the frying pan you'e fried the meat it) with some tomato juice or stock maybe for gravy-base. Carrots can survive long slow cooking, but potatoes can't. They are most likely to fall to bits.
I like to add some potatoes at the beginning because they thicken the sauce as they break down.
Add all of your veg. It will provide some of the sauce but I usually add about a third full of stock, sometimes half, depending on the ingredients.
Don't be tempted to go and stir it a lot. Keeping the steam in is part of the process
The only meat I brown is mince, I just chuck everything in, veg at the bottom, I use about half the liquid, as slow cookers keep all the steam in creating more liquid.
If you have Facebook there are quite a few slow cooker groups, amazing what they can make in slow cookers.
Veg take a lot longer to cook than meat in a slow cooker. I always just put everything in together. As fairycakes says you do not need as much liquid at the start.