Coat the bottom of a large pot* with 2 tablespoons of olive oil and place it over medium heat.
Add the mushrooms in an even layer. Be sure not to crowd the mushrooms — cook them in batches if needed.
Cook the mushrooms until browned on bottoms, about 5 minutes. Flip and cook them for 5 minutes more.
Remove the mushrooms from the pot and transfer them to a plate.
Add the remaining 2 tablespoons of oil to the pot. When oil is hot, add the onion. Sweat the onion until soft and translucent, stirring occasionally.
Add the whiskey and thyme, and return the mushrooms to the pot. Bring the liquid to a simmer and allow it to cook until reduced by about half, about 5 minutes.
Sprinkle 2 tablespoons of flour into the pot, a bit at a time, stirring frequently to form a thick paste that coats the mushrooms.
Once all of the flour has been added, stir in the broth, soy sauce and liquid smoke if using. Stir the mixture until smooth and bring it to a simmer.
Allow the gravy to simmer for about 10 minutes, until the sauce is thick and the mushrooms are tender. Add a bit of water if a thinner gravy is desired. If thicker gravy is desired, transfer a bit of the sauce to a bowl, whisk in a bit more flour to form a slurry, and then whisk the flour mixture into the gravy and let it simmer for a minute more.
Remove the pot from heat. Season the gravy with salt and pepper to taste. Serve.
*If you'd like to save time you can cook the mushrooms in one cooking vessel (I used a cast iron skillet) and start the gravy (step 5) in a separate pot while they cook.