All great dishes start from humble beginnings and every cuisine has their own. I am sure everyone reading this has started a dish with one of these three bases sautéed in butter or olive oil in a 2-1-1 ratio.
Mirepoix: If you cannot guess from the name this is French. It is using the basic ingredients of diced onions, carrots and celery. I use this when starting all my stocks and soups. Typically, you would butter for a mirepoix.
Trinity: When I think of the trinity I think of Emeril Lagasse and his references to it in his Cajun and Creole cooking. This again uses onion and celery but then swaps green peppers for the carrots. I have seen recipes using olive oil, butter or combination of both to sauté.
Sofrito: Lastly, we have the Spanish version using onion, garlic and tomato in olive oil.
These aromatic mixtures will develop your dishes right from the beginning. You will want to add black pepper and a little salt. Be careful with the salt as if you add too much or too early it will get watery. Salt draws out water from the vegetables. If you want to add a sweetness from your base you can start the onions first and then when they start to caramelize add the other vegetables.
Now you know the names of the humble beginnings I am sure you all have been using.
One last tip: If you are reading a recipe and it says to use a medium dice you may be like well what is medium? A medium dice is 1/2 inch X 1/2 inch X 1/2 inch.