I do not recommend grilling the steaks over a flame; I found the porcini rub and flavor burn away. Using a cast iron skillet gave me the best caramelization without losing the porcini flavor.
I purchased my European porcini powder from Far West Fungi in the Ferry Building in San Francisco; you can order from them online or make your own by running porcini mushrooms in a spice grinder.
For my mixed grill, in addition to the steak, I served a few lamb riblets – (simply seasoned with salt and pepper, then grilled a minute or so each side), German-style Weißwurst (which are fully cooked and needed only a warming on the grill, grilled cherry tomatoes, and some shishito peppers.
If you are only making one steak, I suggest keeping the remaining porcini rub in a glass jar in your freezer.