Preheat oven to 400 degrees F (200 degrees C). Grease a baking sheet with cooking spray.
Heat olive oil in a saucepan over medium heat. Cook and stir onion in hot oil until slightly browned, about 5 minutes. Add tomatoes, leek, garlic, and salt; saute until tomatoes are soft, about 5 minutes more. Sprinkle lemon juice over the vegetable mixture. Stir in basil; heat until wilted, about 1 minute.
Arrange portobello mushroom caps onto the prepared baking sheet. Fill mushroom caps with onion mixture. Sprinkle Parmigiano-Reggiano cheese on top.
Bake in the preheated oven until mushrooms are heated through and the Parmigiano-Reggiano cheese has melted, about 12 minutes. Sprinkle with parsley.
