Slowly cooking the onions will caramelize them, which brings out the sweetness and gives them a brown color.
Caramelize Sentence Examples
Carbonized bamboo has been steamed to a point in which the naturally occurring sugars in the wood are "cooked" until they caramelize, turning the bamboo into a warm brown color, similar to that of brown sugar.
Cook them just until the sugars in the fruit caramelize, remove them from the grill and serve them with ice cream.
Adding a little sugar to your sauté pan will help caramelize the veggies for added flavor.