Mix the flour and optional spices if using. Add the mushrooms to the flour and toss well to coat.
Frying
Using a thermometer, heat the oil to 300 degrees in a stock pot or dutch oven note that you will need at least a couple inches of oil in the pan to make it work. Make sure the oil is at least 3 inches beneath the lip of the pan to prevent overflowing.
Remove the mushrooms from the flour in small batches, tapping off the excess, then, working carefully, add them to the hot oil and cook for roughly five minutes (this will vary depending on species) or until you don't hear water sizzling in the oil (this means the water has been cooked off and the mushrooms will stay crisp and not be soggy.)
When the mushrooms are thoroughly fried and golden, remove them from the oil to a bowl and toss with salt to taste. Remove the mushrooms to a cooling rack or paper towel on a tray in a warm oven. Repeat the process with the rest of the mushrooms.
You can hold finished, fried mushrooms in a warm oven until all of the mushrooms are fried or place them on a cooling rack to help them stay crisp.
Make sure that when you lay them out without touching after frying. Don't put them in a pile on top of each other as it will trap escaping moisture and make them less crisp.
Storing
The mushrooms keep well for a day or two after they're cooked, unrefrigerated in a paper bag. Don't store them in a plastic container or they'll become soft. Reheat them in a 300 F oven for a few minutes to make them crisp again. At my house they rarely last more than an afternoon.
Mustard Version
To make the version with mustard and herbs, take the oyster mushrooms and brush them lightly on the gill side with dijon mustard. Sprinkle with a mixture of herbs.
I like a mixture of half Italian parsley mixed with thyme, tarragon, savory or oregano. Proceed with the recipe as directed above for frying the mushrooms, omitting the salt from the flour mixture.
The mustard version contains more moisture, so it's more delicate. You'll need to watch them carefully so they don't scorch.
Notes
Use enough oil
A trick to making these is they need to be completely submerged in the oil. Buy a gallon of frying oil. Strain the oil after cooking and save it. Frying oil can be re-used 2-3 times.
Frying safety
Make sure that the oil is 3 inches below the lip of the pan. This will prevent overflowing when you add the mushrooms to the hot oil.