Clean mushrooms by brushing them lightly or rubbing them with a clean towel. (Note never wash mushrooms as they are like sponges and will absorb the water leaving a spongy soggy mushroom.)
Place them in a large bowl.
Cut the parboiled potatoes in half keep and place in a separate bowl from the mushrooms.
Drizzle the mushrooms with a tablespoon of oil and season with pepper, lemon zest of 1 lemon and 1 tablespoon of the garlic. (Note: never salt your mushrooms before grilling it again extracts moisture within the mushroom and will not give you a nice caramelized mushroom as the moisture will start to steam when heated as opposed to searing.
Drizzle the potatoes with a tablespoon of oil and season with lemon zest of 1 lemon, garlic salt, pepper.
Preheat one side of the grill to 350°F (175°C) medium heat.
Preheat a grill basket on the barbecue over medium high heat (375°-400°F) (190°C-205°C).
Oil the grill grate and grill basket to help avoid sticking.
Place potatoes cut side down on the grill and cook turning occasionally for approximately 10-15 minutes until crispy and golden brown.
Place the mushrooms in the basket and cook for 8 minutes or less while turning and flipping until nice golden char marks are achieved.
Remove from the mushrooms from the grill and place in a bowl. Season with salt.
Remove the potatoes from the grill and place them in a bowl. While they are still hot, smash them lightly with a fork and drizzle the remaining olive oil and warmed butter overtop.
Gently fold the golden mushrooms into the bowl of potatoes. Add the basil and chives and fold gently.
Serve this mixture with sour cream if desired.