Bring a large pot of salted water to a boil.
Cook the linguini according to package directions until al dente. Just before draining, carefully scoop out and reserve about ½ cup / 120 ml (4.1 fl oz) of the starchy pasta water.
Drain the pasta and return it to the pot or a large bowl.
While the pasta cooks, prepare the mushrooms. Trim the ends of the king oyster mushrooms and slice them lengthwise into ½-inch / 1. 25 cm thick planks.
In a bowl, toss the mushroom planks with olive oil, lemon juice, minced cilantro, basil, red pepper flakes, and a pinch of salt and pepper until evenly coated.
Thread the mushrooms onto metal or pre-soaked wooden skewers.
To make the sauce, combine the avocados, olive oil, lemon zest and juice, spinach, cilantro, basil, garlic, salt, and pepper in a food processor. Pulse until chunky. With the processor running, stream in the reserved pasta water, 1 Tbsp / 15 ml at a time, until the mixture is creamy and reaches your desired sauce consistency. You may not need all the water.
Heat a grill or grill pan over medium-high heat.
Grill the mushroom skewers for 4-5 minutes per side, until tender with defined grill marks.
Pour the avocado sauce over the warm pasta and toss thoroughly to coat.
Serve immediately, topped with the grilled king oyster mushrooms.