In a zip lock bag add 2 tbsp soy sauce, 1 tbsp Worcestershire sauce and balsamic vinegar, 1 tsp garlic powder, 1/2 tsp salt and 1/4 tsp black pepper. Give it a stir with a fork to blend, then add in your mushrooms and thyme. Zip the bag shut and give it a good swish around with your hands.
Leave to marinate for 30mins - 1hour, shaking about every now and then to distribute the marinade properly. Careful, the mushrooms will shrink considerably (due to salt/vinegar) if lest for a long period of time, so just keep an eye on them.
Set up 3 bowls: 1st is your flour with a pinch of salt and pepper, 2nd is your beaten egg and 3rd is your Panko with a pinch of salt and pepper.
Take a few mushrooms, shake off the marinade then pop in the flour, then egg and finish in the breadcrumbs. Really ensure you fully coat each mushroom at each stage, especially the last one (Panko). Repeat with all mushrooms.
Heat up oil in a deep pot/pan to 180C/356F. In batches carefully lower the mushrooms into the oil on a slotted spoon. Allow to fry for 2-3mins, or until nicely golden. Stir them about a bit to ensure they get an even fry. Carefully remove with a slotted spoon and place on a cooling rack with paper towels underneath (don't place directly on paper towels or they steam and go soggy).
Enjoy with your favourite dip (ideas in notes section).