Most traditional recipes for pho call for charred onions or spices to bring an intense smoky flavor to your bowl of pho. Not every takes this step, especially if you are pressed for time and looking to make a quick version of pho.
For the purists, this is how you char your onions for pho.
Basically, you want to lightly char or burn the outside of your onion, or whatever ingredient you are using to bring out more flavor. Onions are the most common ingredient to get charred, but you never know what the recipe will call for. Sometimes charred ginger.
Use tongs to hold the item in an open flame, like the flames on a gas stove or even on a BBQ grill. Setting the ingredient directly on a hot electric burner coil can also work if it’s clean. Let the piece get nicely singed around the edges, but try not to let the whole thing go up in flames. That’s not good. Should only take a few minutes.
If you really don’t have decent access to an open flame, you can also use your oven broiler. It will take longer, closer to 10 or 15 minutes to get charred.
Once nicely charred, you usually just add directly to your pho broth, after you remove any really black or burned bits.
So next time you try a pho recipe, char your onion or ginger for a really traditional broth flavor.
On a similar note, you can also bring out the flavor of whole spices (cloves, anise, coriander, cinnamon) by toasting them in a hot, dry pan before you add them to the broth.