Make pesto: Place basil, arugula, pine nuts, cashews, lemon juice, parmesan (optional), salt, pepper, and garlic in a food processor or blender. Process (scraping down sides occasionally) until almost smooth. Slowly pour in olive oil in and blend/process until all oil is combined. Transfer pesto to a bowl. Set aside
Preheat the broiler (to high) with a rack about 8 inches from the heating element.
Place the sea bass on a baking tray lined with aluminum foil or parchment paper. Use a clean cloth or paper towel to pat the sea bass dry.
Spread a 1/4-inch thick layer of pesto on top of the sea bass.
Cook under the preheated broiler until a crust forms and fish flakes easily with a fork, about 10-12 minutes. (If the crust begins to blacken, tent with foil.)
Remove the sea bass from the oven. Squeeze juice from 1/2 lemon over the fish.