In a large bowl combine the oil, vinegar, salt and pepper. Add the spinach and onions and toss to combine. Divide the salad between 4 plates.
Place a ball of mozzarella atop each portion of salad. Distribute the red pepper slices over each salad if desired. Let each person slice into their ball of burrata.
Notes
Burrata is a semisoft white Italian cheese made from mozzarella and cream. I think of it like the cheese cousin to a molten chocolate cake. The outside is just the softest silkiest mozzarella, and when you cut it open, a rich creamy filling rolls out of the middle. You must have lovely bread to scrape every bit of the cheese onto.