Slice the red onion into 1/4 inch slices.

Remove stems from shiitake mushrooms and clean them.

In a bowl, combine peanut oil, rice wine vinegar, sesame oil, minced garlic, roughly chopped scallion, salt, and pepper. Mix well to create a marinade.

Add the sliced onions and shiitake mushrooms to the marinade and mix to ensure they are well coated.

Let the mixture sit at room temperature for at least 30 minutes to allow the flavors to meld.

Preheat the grill to medium-high heat.

Grill the mushrooms until they have grill marks and are soft.

Grill the onions until they are tender and starting to caramelize.

Place the grilled mushrooms and onions back into the bowl with the remaining marinade and toss to coat.

Serve the salad warm.