I don’t know if these are well known, if they aren’t, give them a try.
Tomato sauces, if they are a bit bland try adding a dash of soy sauce.
If you want a ‘Beefier’ taste in a dish, add Vegemite/Marmite, it’s all umami and really helps. You can make Vegetarian French Onion Soup using it and meat eaters will love it.
Mushroom soup/ sauces, add cinnamon (preferably cassia, not true cinnamon). Just a small amount, below the threshold of where you can identify it as an ingredient. Iirc something about the phenols in the cassia boosts the mushroom flavors.
ETA: based on another comment, chocolate sauces/ Ganache, add vanilla and espresso. If you want the most delicious Reese’s peanut butter cup cheesecake, ice cream etc. You want a shot of espresso and a touch of vanilla.
If you have not tested yourself to see if you are a Supertaster, you should. It makes a world of difference when you are cooking for other people.
If you have others please add them, these are some of my favorites.
While baking bread, specifically sourdough or similar lean doughs, don’t just use 100% bread flour. Replacing ~20% or so with whole wheat flour, rye flour, semolina, or similar more flavorful flours makes a big difference in the final flavor of the bread. If you are using whole wheat, just keep in mind that it goes bad fairly quick due to the fat content in the germ, especially if you just keep it in the paper bag it came in. Just give the flour a smell before you use it; it should smell fairly neutral or good. Too many people will use whole wheat flour that’s been sitting in their cupboard for 5 years and gone rancid.