> If the recipe calls for chopped onions, chop them before you turn on the burner. Later you can work on parallelizing the process
For most meals I start the slowing-cooking item first and then start on the others once it is in the over / pot or similar. E.g. a roast joint. I'll get it in the oven and then prep the roasting veggies (e.g. potatoes and parsnips), get them in the oven and then start peeling the green veg (which only need cooking for a few minutes). Over the years I've learned how long things need and how long I take to prep them, so my timings usually work out.
Chinese cooking (or at least my take on it) is my main exception: given the short cooking times of some things, and the large number of ingredients / dishes, I tend to prep everything first, and then start cooling simultaneously.
The other obvious exception is using my smoker, where a meal might stay in it for more than 5 hours. I say 'meal' rather than 'meat' because I also use the smoker to cook chilli and tagine.
Overcooking meats to be on the safe side, and overcooking the veggies, because they are in with the meat for the same amount of time, but they should be in for less, etc..
I guess I'm missing out on taste and nutrients aswell due to this?
While I was unemployed for 9 months I got a brilliant idea to learn cooking outside of sandwiches and very basic meals. One of the main dishes I'd wanted to learn was boiled rice which everyone around me always fail to do properly except SEA expats. I was surprised that for each shape and size of rice I could buy there are 2 or 3 different sources and/or cultivars which behave differently but people tend to cook them the same way.
offtopic: where you got Space Rangers from?
Totally unrelated, but I have a dvorak keyboard layout, and navigation using HJKL is one of the things I hate most about vi. Sure, I could rebind it locally, but that doesn't help me when I use a remote machine. Perhaps this is what being left handed feels like.
Take mise en place seriously. If the recipe calls for chopped onions, chop them before you turn on the burner. Later you can work on parallelizing the process (eg, chopping the onions while the bacon is rendering) but to start with, just work on getting all the pieces on the table before you start the chess clock. Delegate to sous-spouses or sous-children.
Rarely do you need to be a perfectionist. If you are sautéing or stewing or braising or souping, the onions cook down. Ceviche is a different matter. Baking may be more specific, but that kind of witchcraft is forbidden around here.