Clean your mushrooms gently with a damp paper towel and trim the stems if needed.
Let them dry out before you do anything else—seriously, this helps the breading stick.
Mix the flour, salt, pepper, and dry mustard in a medium bowl until it looks even. I always double-check for lumps before moving ahead.
Pour the buttermilk into another bowl. Dip each mushroom in buttermilk so it’s coated all over, then shake off the extra.
Roll the mushrooms in the seasoned flour mixture, making sure each one gets a solid coating. I press the breading on a bit so it doesn’t fall off in the oil.
Heat about two inches of vegetable oil in a deep skillet or pot over medium-high until it hits 350°F. Test it with a thermometer if you have one—it’s more accurate than guessing.
Fry the mushrooms in batches, turning as needed, for about 3-4 minutes until they’re golden brown all over. Don’t overcrowd the pan or they won’t cook evenly or stay crisp.
Set the fried mushrooms on a wire rack or paper towels so any extra oil drips off. Let them cool for just a minute or two before serving.