Soak the cashews overnight in cool water, or cover them with water in a saucepan and boil for 15 minutes. Drain.
Make the dressing. Add the soaked and drained cashews to a high-powered blender or food processor. Add the remaining ingredients and blend for a few minutes, scraping down the sides as you go, until the dressing is thick and creamy. If too thick, add a spoon or two of water. Season to taste with more salt and pepper or lemon juice as needed.
Make the pickled onions (optional). Bring the water to a gentle boil. Place the onion slices in a large mason jar. Mix together the hot water, vinegar, maple syrup, salt, and red pepper flakes. Stir to combine until the syrup is dissolved. Pour the pickling liquid over the sliced onions in the jar. Allow to cool to room temperature, at least 30 minutes. You can serve now, but I prefer to refrigerate them for a more developed flavor. Grill the romaine. Preheat the grill (medium to medium-high) with the lid on. Season the grill as needed.Note: If using a grill pan, heat it to high.
If using whole romaine lettuce heads instead of romaine hearts, remove a few of the outer layers and save for another salad or use. Rinse and dry the lettuce. Cut each romaine heart in half, lengthwise, leaving the core intact.
Drizzle each side of the romaine hearts with some of the avocado oil and rub it into the nooks and crannies. Sprinkle with some kosher salt to season.
Place the romaine, cut-side down, on the grill. Cook for 3 to 6 minutes, turning occasionally, until the leaves are nicely charred and grill marks form. If the grilled lettuce seems to start sweating water, transfer it to a colander or wire rack for a minute or two to allow excess moisture to drip off.
Arrange grilled romaine on a platter, cut-side up. Scatter pickled onions on top and drizzle with the dressing.