• Heat the olive oil in a medium nonstick skillet. Add the onions, cover the pan and cook, stirring occasionally, until the onions are deep golden brown. During the cooking, add water as necessary to stop the onions from burning and drying out. Be sure that all of the water is absorbed before removing form the heat.
• Place the potato halves in a large saucepan and cover with cold water, 1 inch above the potatoes. Salt the water well. Bring to a boil and cook until the potatoes are tender when pierced with a fork, 20 to 25 minutes. Drain well and transfer to a large bowl.
• Preheat the oven to 350 degrees F. Lightly coat an 8- by 8- glass or ceramic baking dish with cooking spray.
• With a potato masher, mash the potatoes until the mixture is a smooth as possible (it won’t be completely smooth because of the skins). Add the butter, cream cheese and milk, and continue to mash until combined. Season with salt and pepper.
• Stir in the caramelized onions, crumbled bacon and rosemary.
• Transfer the potatoes to the prepared baking dish, smoothing the top with a spatula.
• Sprinkle the cheese over top. Bake until the casserole is warmed through and the cheese is melted, about 25 minutes.

Make ahead directions:

• Follow the directions above, through step 6. However, be sure to add additional milk, about ¼ cup. The mashed potatoes should have a looser texture than desired because the potatoes will absorb the extra liquid while refrigerated.
• Allow to cool completely, cover well with plastic wrap and refrigerate.
• Remove the casserole from the refrigerator 1 hour before baking. Remove the plastic wrap, sprinkle the cheese on top of the casserole and cover with foil.
• Reheat the casserole in a 350 degree oven until the casserole is heated through, 45 to 50 minutes. Remove the foil for the last 10 minutes to allow the cheese to melt and brown slightly.