I was reading this column in the Guardian, where Yotam Ottolenghi and Noor Murad are answering questions from readers. One answer which made me pause is this one from Yotam Ottolenghi on key skills to sharpen when starting out as a cook:  There are certain processes so fundamental to cooking that you want to get them right: when you saute onion and garlic, how far to take it and what you are going to get with different temperatures – low and slow for sweetness. A dish can stand or fall on how you cook your onions at the beginning. It’s amazing how important it is.  So, I guess I kinda get what he's going for - depending on how long you cook onion for, it will affect how sweet it is. Similarly it matters if you add a bit of vinegar and so forth. At the same time, I feel he is being frustratingly vague about exactly what skills it is you need to sharpen for getting better at cooking your onions and garlic. Can some some chefs more experienced than me please explain in some more depth what key skills it is important to develop for cooking onion and garlic? 
My interpretation is that this is a combination of knowing ingredients and technique.  How you treat ingredients like onion and garlic, whether you cut, slice, crush it...or turn it into a paste, will dramatically impact the dish.  In addition, how you cook those ingredients, whether high heat, and quick to brown, or very low and gently will also greatly impact the results...and there are all sorts of permutations on just those few variables for just those ingredients...add more ingredients and more variables...it can get complicated quickly.  However, I believe his point is that building your cooking knowledge from a basic understanding of ingredients, and what happens when you treat them differently is a critically important foundation, upon which you build the flexibility to be creative. My advice-- play with onions and garlic...observe the difference in flavor as you treat and cook them differently...low and slow, high heat with browning...adding ingredients near the end.... How do you want the flavor to be noticed...background...up front....bright...muted?  Notice the impact on your final dish. You just have to get a feel for how your ingredients behave so that you know how to get the result you are looking for. 