Heat a sauté pan on medium heat, add olive oil and onions. Allow to slowly soften for about 10 minutes.
Evenly spread cottage cheese on flatbread. Add onion mixture, then plums and sprinkle with goat cheese.
Toast or bake in a toaster oven or regular oven at 400 degrees, directly on rack until cheese is partially melted and starting to bubble.
Top with a drizzle of balsamic vinegar.