Heat butter in a large skillet over medium-high heat. When the butter is melted, add the onions. Toss with a spatula until the onions are fully coated in the butter.
2 tablespoons butter, 2 large red onions
Reduce heat to medium-low and cook, stirring occasionally, for about 30 minutes, or until soft and golden.
In a medium bowl, add the tomatoes, olive oil and salt and mix well.
1 cup cherry tomatoes, 2 tablespoons olive oil
In each serving dish, stack the tomatoes, bocconcini, basil leaves and caramelized onions. Top with balsamic vinegar and serve.
1 cup fresh bocconcini cheese, ½ cup fresh basil leaves, 4 teaspoons balsamic vinegar