In a small skillet, add the onion, olive oil, salt, and crushed red pepper. Turn the heat to low, and let the onions very slowly cook for 30-45 minutes, stirring every few minutes, until the onions are very dark brown and soft but not black at the edges.

Preheat the oven to 450F. Build the flatbreads by layering each with half the apple butter, onions, walnuts, apple slices, sharp cheddar, and blue cheese crumbles. Place on a baking sheet, and bake for 10-15 minutes until the cheese is bubbly. Let cool for 5 minutes on the baking sheet, then cut into wedges, top with the watercress, and serve!