1. 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.
2. 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.
3. Spoon cashew puree into mushroom caps. Top with corn, spinach, scallions, and any remaining marinade
Meaty vegetables, like Portobello mushrooms, can make a light, satisfying dinner, even for those with bigger appetites.