Prepare a charcoal grill or gas grill, heating to 400 degrees Fahrenheit.
Remove the stems from 2 8 oz. packages of cremini (also known as Baby Bella) mushrooms and 1 8 oz. package of white button mushrooms. Discard the stems in the compost bin.
Wipe any dirt from the mushrooms using a lightly dampened towel (don't wash the mushrooms with water, or they will become saturated and rubbery when cooked).
Cut any large mushroom in half, making sure that all of the mushrooms are relatively the same size so that they cook evenly.
Place the mushrooms onto metal skewers, making sure there is a little room between each mushroom (there should be 4-5 skewers in total). Measure 3 Tbsp. of olive oil and brush both sides of the mushrooms with it.Measure ½ tsp. each of kosher salt and pepper and season both sides of the mushrooms with it. Place 2 oz. of chopped, unsalted walnuts (or unsalted nuts of choice) into a small cast iron skillet. Peel 4 cloves of garlic, place them onto a small piece of tinfoil, and drizzle with a little olive oil. Wrap the garlic cloves tightly in the tinfoil.
Once the coals are hot, move them over to one side of the grill (this will create direct heat). Place the mushroom skewers over the direct heat.Place both the cast iron skillet and tinfoil package one the other side of the grill over the indirect heat. Place the lid on the grill and cook for 5 minutes. Flip the mushroom skewers, return the lid to the grill, and cook for another 5 minutes. Stir the walnuts at the 5-minute mark to make sure that they are not burning.
Remove the mushroom skewers, cast iron skillet, and tinfoil package from the grill and allow to cool.
Mince 2 Tbsp. of fresh chives. Parsley is another great option if you prefer.
Remove the grilled mushrooms from the skewers and place them into a food processor along with the toasted walnuts and roasted garlic. Add 1 cup of pitted Kalamata olives, 2 tsp. Dijon mustard, 2 tsp. red wine vinegar, 3 Tbsp. lemon juice, ¼ tsp. each of kosher salt and pepper, and 2 Tbsp. of minced chives, parsley, or fresh herbs of choice. Pulse the food processor just until all of the ingredients come together (try not to over-pulse it, or the texture may become too mushy).
Place the tapenade into a serving bowl. Garnish with the zest of 1 lemon (a Microplane or small grater works great) and some extra minced chives or herbs of choice. To Serve: Serve the tapenade with grilled baguette, crostini, or bread of choice. Sprinkle with balsamic glaze of a sprinkle of flaky sea salt for even more flavor (optional). It can also be served with crackers or raw veggies, spread onto a sandwich, or used as a topping for pasta or roasted or grilled meats.