Preheat the oven to 350 degrees F.
Make the cheesy sauce by combining all ingredients and blending into a really smooth paste. Set aside.
Cook the potatoes.In a microwave: Place the diced potatoes in a microwave-safe dish and add 1 tbsp water. Cover and nuke on high for five minutes or until potatoes are tender. On the stovetop: Place the potatoes in a saucepan, cover with water by an inch, then bring to a boil. Cover, turn heat to a simmer, and cook the potatoes 10 minutes or until very tender. Drain the water and reserve the potatoes.
In a large skillet, heat the oil. You can do this in a 10-inch cast-iron skillet or any skillet that can go from the stove to the oven, to reduce cleanup.
Add the onions, garlic, parsley and rosemary (or sage) to the oil. Season with salt and ground black pepper and saute, stirring, for five minutes over medium heat or until the onions are soft.
Add the sausage and potatoes and continue to cook, about five minutes. Check seasoning and add more pepper and salt if needed.
If you are using a cast iron or a stove-to-oven skillet, arrange the potato mixture in an even layer and pour the cheesy sauce over it evenly. If using a baking dish, like I did, transfer the potato sausage mixture to the baking dish and pour the cheesy sauce on top.
Transfer the skillet or baking dish to the oven and bake 30 minutes or until the top starts to lightly brown. Switch to the broil function on your oven and broil for another five minutes on the highest setting, or until gold-brown spots appear on the potatoes and cheesy sauce.
Remove from the oven, garnish with more parsley if you wish, and serve hot or at room temperature.