I put garlic in just about everything. Probably not really a secret ingredient, but it does go with just about everything

For soup I like to add turmeric for the colour. Also kinda tasty

Little bit of vinegar and MSG will liven things up in many cases

Marmite in stews

That stuff is basically pure msg, so a spoon full in a pot will add a whole lot of taste without making it taste like marmite

I don't like the stuff of toast, but it's nice to have a glass around for cooking

Boullion and smoke seasoning. Boullion makes the meat, especially chicken taste more well rounded. Smoke season I will add when using an indoor grill/skillet and it will really taste like it was done on an open flame.

Soy sauce in lieu of salt, for pretty much anything liquidy. Gravy, soup, pasta sauce, what have you.

I put a little ginger in just about everything.

Grilled meat. Ginger.

Brownies. Ginger

Pasta

Yeah a little ginger in that too. Why not.

Tea. Yeah a little sprinkle.

HonDashi. I add it to nearly all my soup stocks (unless I'm trying to make a dish vegetarian).

I add beetroot. This makes the soup look discusting but it tastes good and is full of iron - which was the main purpose of including it.

Salt. people don't use enough salt.

I prefer a splodge of mustard in my potato salad. And I second the suggestion of adding garlic to most everything, including veggie soup.

Soup is about just throwing in whatever you have. Generally, if there's some kind of a meat/bones, gelatin will give the broth body. If it's more of a bean or potato situation, you may need to pull a portion out, mash it up, and add it back in to give body. You could also temper some eggs and add them in.

If you dont have gelatin or something starchy, you can add a cornstarch slurry to thicken it. If you have a really thin broth, it won't taste right even if it would otherwise be really tasty. Taste is an amalgamation of senses, and texture is part of that.

Acidity definitely helps soups. Brothy beans are great with a little vinegar, some soups are good with lemon, etc.

I have a black eyed peas soup recipe with a secret ingredient. For every pound of peas I use I add 2 pounds of pork.

Anchovies

A little cinnamon is great on chicken or in rice.

I am a huge fan of Bragg liquid aminos

For some reason it tastes, to me, the way that I always wanted soy sauce to taste.

A little bit salty, a little bit umami, and it's a great thing to add onto meat dishes or soups.

It's also very good for sauteing mushrooms, just saute the mushrooms in butter, throw in some braggs, and then once the mushrooms are cooked, pour in a little red wine to deglaze the pan, and then cook that down until the red wine has evaporated.

Makes the most delicious mushrooms I've ever had in my life.

When I don't know what's missing a dash of lemon or lime juice solves the problem. Acid fixes issues of depth, salt fixes flavors not coming through properly

For potato salad: I have several secret ingredients. I don't necessarily use them all at the same time, but hard boiled eggs, yellow American mustard, and relish made from home-made sweet pickles.

On the topic of pickles: This is particularly true for sweet pickled veggies (like cukes & peppers), I'll add some fresh cut ginger. Pairs extremely well with more the more traditional pickling spices used in my part of the world.

For most anything savory: A bit of celery seed. This one falls into the "with great power comes great responsibility" category, though. Use just enough that you, the cook, can detect it but not so much that other people can easily pick it up in the flavor profile. Also, it works best with dishes that pair sweet and salty profile with herbs, such as soups and marinades.

For dishes that need a bit of extra fat: I'll sometimes add some bacon grease if I know that everybody having the dish is fine with pork products. It's a quick way to add saltiness, meaty/umami, and smoke flavoring to things like various greens, various bean dishes, and stuff like that.

Fennel.

When I'm making vegetable soup, I usually add some balsamic vinegar, tomato paste, and MSG. The acid and umami make the soup taste more balanced and hearty. I hope it's helpful!

I make a dish with vegetables, chickpea and chicken in the slow cooker with mostly a "Mediterranean" style seasoning, garlic, olive oil, parsley, etc, but I add a touch of soy sauce. It might work for your vegetable soup as well depending on what's in it.

I put layers of sliced zucchini in my lasagna. No need to precook them.