In a bowl, whisk lemon juice and zest, oil, garlic, and rosemary. Season with salt. Arrange mushrooms, rounded side down, in a glass baking dish. Pierce each with the tip of a knife. Drizzle with marinade. Cover and refrigerate for at least 4 hours or up to overnight.
Soak cashews in warm water for 30 minutes. Drain. Pulse in a food processor with 1/4 cup water until smooth. (Add more water for desired consistency.) Season with salt.
Spoon cashew puree into mushroom caps. Top with corn, spinach, scallions, and any remaining marinade.