Begin by thinly slicing 2-3 medium yellow or sweet onions. Gather your olive oil, salt, and optional additions. Set stovetop to medium heat and grab a large skillet.

Add about 2 tablespoons of olive oil to the preheated skillet. Once hot, add the sliced onions and a pinch of salt. Cook for approximately 10-12 minutes until onions turn soft and translucent.

Reduce the heat to low and continue cooking for another 20-30 minutes, stirring every few minutes, until onions become beautifully golden brown and deeply caramelized.

When onions reach a deep golden color, add a splash of balsamic vinegar. Stir well and cook for another 2-3 minutes until it evaporates.

Fold the caramelized onions into your casserole mixture, ensuring every bite of your casserole is bursting with sweet onion flavor.