Preheat oven to 400 degrees. Spray a baking sheet with nonstick spray.

Place mushroom cap on the sheet, rounded side down. Bake until tender, about 15 minutes. Remove sheet, but leave oven on.

Meanwhile, bring a medium pot sprayed with nonstick spray to medium-high heat. Add spinach and salt. Cook and stir until wilted, about 1 minute. Remove from heat, transfer spinach to a layer of paper towels, and blot away excess moisture.

Clean pot, and fill with 2 inches of water. Add vinegar, and bring to a boil. Once boiling, lower temperature until a steady simmer is reached.

Crack egg into a small shallow bowl. Give water a stir, and gently add egg. Cook until egg white is mostly opaque, 3 - 5 minutes (3 for a runnier egg, 5 for a very firm one).

Using a slotted spoon, carefully transfer egg to a layer of paper towels to absorb excess water.

Blot excess moisture from mushroom cap, and top with cheese slice. Bake until cheese begins to melt, 1 - 2 minutes. Transfer to a plate, and top with tomato, spinach, and egg. If you like, sprinkle with black pepper.