Should I fry mushooms before adding to a red meat sauce? I'm planning on making a pressure cooker sauce using tomatoes, ground beef, sausages and wine — a classic Italian American spaghetti sauce. The method is to sweat onions, carrots, celery and garlic whilst frying off meat in a separate pan and adding in along with liquids before an hour in the pressure cooker.

I am thinking of adding mushrooms for flavour and texture — my plan was to dice them and cook them with the meat. Will I get better flavour and texture by frying the mushrooms beforehand, or should I just put them in when I sweat the vegetables?
You are right on track with pre-cooking/frying the mushrooms instead of dumping them just into the dish. I think they benefit greatly from it, especially flavor-wise, which gets a lot more concentrated. Whatever you choose to do, remember that mushrooms will exude a lot of liquid during frying. 

So if you add a significant amount to your meat, it will likely interfere with the browning. The same is true if added to your vegetables from the beginning, but it won’t matter as much.

I recommend you either 


brown your meat, then add the mushrooms and use the liquid for a first „deglaze“ or
sweat your vegetables and add the mushrooms when they are like half-done.


Of course you can fry the mushrooms separately, but why bother with an extra step?