Add butter to a large heavy skillet; let melt over medium-high heat.
Add onions into the skillet, cook stirring until tender (5 min).
Turn heat down to low. Add rosemary to the onions, season with salt.
Cook stirring for 30 minutes or until the onions are tender and golden.
Grind some black pepper on top, mix to combine.
Mix in pine nuts just before using.
Use as pizza or pie topping: spread on top of the pizza dough, then sprinkle with blue cheese and halved Kalamata olives or sliced radishes and arugula to complement the sweet taste of the topping with some tartness.