Garlic cooks quickly once it is chopped finely. If it enters a hot pan with a large pile of raw onion, it may become dark and bitter long before the onion is ready.
For many simple sauces, soften the onion first, lower the heat if necessary, then add garlic and stir briefly before adding tomatoes, stock, or another moist ingredient. Larger slices tolerate a different rhythm from a fine paste.
Use your nose as well as your eyes. A pleasant garlic aroma arrives before deep browning in these dishes. If the pan is too hot, take it off the burner briefly rather than racing to add more ingredients.
Burnt garlic is difficult to hide. For a small amount at the beginning of cooking, starting that step again may save more food than carrying the bitterness through an entire sauce.