Preheat oven to 425° F. Slice onion and mushrooms; halve tomatoes and coarsely chop spinach.
Heat oil in a large sauté pan. Add onion and mushrooms; cook approximately 10 minutes until onion begins to caramelize, stirring occasionally to prevent sticking.
Spread sauce evenly onto each Naan bread.
Add cooked onion and mushrooms, tomatoes, and spinach. Sprinkle on feta and season with salt and pepper to taste.