1. Preheat your oven to 400 F (200 C). If your flatbread requires pre-baking, follow the package instructions. Otherwise, place the flatbread on a baking sheet.

2. Evenly spread the sugar-free marinara sauce or pesto over the entire surface of the flatbread, leaving a small border around the edges for the crust.

3. Sprinkle half of the shredded mozzarella cheese over the sauce. This helps to create a delicious base and prevents the vegetables from making the crust soggy.

4. In a small bowl, toss the sliced bell peppers, red onion, chopped spinach, and sliced mushrooms with olive oil, oregano, salt, and pepper. Arrange these seasoned vegetables evenly over the cheese layer on the flatbread.

5. Sprinkle the remaining mozzarella cheese over the vegetables. This will help to hold them in place and create a golden, bubbly topping.

6. Place the baking sheet in the preheated oven and bake for 15-20 minutes, or until the cheese is melted and bubbly, and the crust is golden brown and crisp. Keep an eye on it to prevent over-browning.

7. Carefully remove the flatbread from the oven. Let it cool for 2-3 minutes before slicing and serving. This short cooling period helps the cheese set and makes for easier, cleaner cuts.