Place the bocconcini on a parchment or wax paper lined tray and cover it with plastic wrap. Then freeze it over night r at least 6 hours until the cheese hardens completely.
In a bowl add the flour.
In a separate bowl, add the pesto, balsamic vinegar, egg and a pinch of salt.
In a third bowl, mix the pistachios and panko bread crumbs.
Preheat an electric fryer or a small pot filled half way with oil till 375 degrees F.
Take each bocconcini ball and dredge it in the flour bowl, shake off any excess. Then drop it in the basil pesto mixture and roll it well to coat, and finally roll it in the pistachio bowl and make sure the entire surface is covered well with the coating. Place on a wax paper lined plate.
Repeat with all the bocconcini, and if you’d like to fry mushrooms, repeat the same procedure with the mushrooms too.
Fry the bocconcini and mushrooms for just 1 minute and maybe 30 seconds extra (depending on the temperature of your oil and how crowded your pot is). Drain on paper towels and sprinkle a pinch of salt. Serve.