Preheat oven to 170°C. Brush a round 20cm (base measurement) cake pan with 1 tablespoon of the olive oil to lightly grease. Line base and side with non-stick baking paper.
Place potato in a saucepan and cover with cold water. Bring to the boil over medium-high heat and cook, uncovered, for 10 minutes or until tender. Remove from heat and add spinach. Stir until spinach wilts. Drain and refresh under cold water. Use your hands to squeeze as much excess liquid from spinach as possible, then coarsely chop. Set potato aside for 10 minutes or until cool enough to handle. Peel and thinly slice crossways.
Heat remaining oil in a frying pan over medium-high heat. Add mushrooms and garlic and cook, stirring, for 5 minutes or until tender. Whisk eggs and sour cream in a jug. Season with salt.
Place half the potato in the prepared pan. Top with chopped spinach and half each of the mushroom mixture and basil. Repeat with remaining potato, mushroom mixture and basil. Pour over the egg mixture and sprinkle with parmesan.
Bake for 45 minutes or until set. Remove from oven and set aside for 15 minutes to cool slightly. Invert onto a plate, cheese-side up. Serve at room temperature with mixed salad leaves.