Combine the cashews and 1 tablespoon olive oil in a bowl; toss to coat the cashews in the oil. Spread onto a baking sheet.
Toast the cashews in the preheated oven until golden brown and fragrant, shaking the baking sheet occasionally, 5 to 10 minutes. Watch carefully so they don't burn. Set aside.
Heat the 2 tablespoons olive oil in a skillet. Cook and stir the onion in the hot oil until the onion softens, about 5 minutes. Stir the garlic into the onion, and cook for 1 minute more before stirring the kale into the onion and garlic mixture. Place a cover on the skillet and cook, stirring occasionally, until the kale softens, about 7 minutes. Stir the spinach into the mixture, season with the sea salt and white pepper, and continue cooking until the spinach wilts, about 3 minutes. Toss the cashews with the mixture to serve.