Dry onion soup mix is called for as an ingredient in recipes so often, it's easy to wonder if anyone ever even uses it to make soup! Soup mixes like this aren't always the most common ingredient to keep in stock in a kitchen, so it's not strange to find yourself in the position of needing to substitute with something you're more likely to have in stock. How best to replace the onion soup mix depends on the type of recipe it's needed for.
In dishes like chip or vegetable dishes, or any recipe that calls for onion soup mix to flavour a creamy base (like sour cream, cream cheese or mayonnaise), you can usually substitute onion powder for dry onion soup mix and end up with a flavour similar to what was intended. This works best if the recipe includes other herbs and flavours besides the onion soup mix. If not, mix in a generous pinch each of chives, salt and white pepper.
Use garlic powder as a flavouring agent in creamy sauce and dip recipes instead of onion soup mix or onion powder, or use it along with onion powder in equal measure. This will give you a different flavour, but one that's equally pungent and likely to go well with anything the onion dish would have matched up with. Likewise, flavoured garlic mixes are a good substitute, but be cautious about how much you add, since these can be quite salty.
Fresh Onion and Salt
Substitute about a quarter cup of fresh onion and a teaspoon of salt in any slow-cooked, savoury dishes that call for a packet of onion soup mix as an ingredient. Add the onion along with other vegetables to give it time to cook; compared to powdered seasonings, fresh ones need more time to soak their flavour into the dish.
Meat Soup Broth
In many dishes, soup broth bases can be used interchangeably. Each type of broth will give the finished recipe a bit of a different taste, but unless the broth flavour is the main focus of the dish, you'll still end up with something that works well. This substitution is best for dishes like complex soups. Use chicken or pork broth if you want a light flavour, but use beef if the dish has other onion ingredients that are prominent; beef broth is a main component of traditional French onion soup, so they go well together.
If you need a broth substitution but don't want a meat-based broth, go with any powdered vegetable broth. To reproduce the taste of onion soup mix in a recipe, however, you'll want to look at the ingredients and find a broth that's heavy on the onion and garlic, or on similar ingredients like leeks or shallots. If the broth seems to have more of ingredients like carrots or greens, your results may not be quite as good.