Preheat a grill to medium and brush the grates with vegetable oil. Combine 1/2 cup olive oil, 2 tbsp chopped fresh parsley, 2 tbsp chopped fresh cilantro, 2 tbs chopped scallions, 2 tbsp red wine vinegar and 1 clove garlic in a blender and pulse until combined. Add 1/4 tsp red pepper flakes and season with kosher salt. Pour half of the herb oil into a bowl; set aside for serving. Toss the swordfish (2 pounds center-cut swordfish, skin removed, cut into 1-to-1 1/2-inch pieces) with 1 tbsp olive oil, the 1 1/2 tsp ground cumin and 1/2 tsp ground coriander in a bowl; season with salt. Thread the swordfish, 4 ounces shiitake mushrooms, stemmed and halved or quartered if large and 1 red onion, quartered onto eight -10-inch skewers. Grill the kebabs until just marked, about 2 minutes per side, then continue cooking, turning and brushing with the remaining herb oil, until just cooked through, 5 to 6 more minutes. Drizzle with the reserved herb oil.