Preheat oven to 425 degrees.
Pierce potatoes several times with a sharp knife or fork. Liberally sprinkle salt on wet skin. For softer skin, rub potatoes with oil, sprinkle lightly with salt and pepper and wrap with foil.
Place potatoes on a baking sheet and cook 45 to 60 minutes, until a knife inserted in the middle meets no resistance.
Meanwhile, heat a drizzle of olive oil in a pan and sauté spinach until just wilted.
Remove potatoes from oven and lower heat to 400 degrees. Let potatoes cool, then cut in half lengthwise. Scoop out the centers, leaving about ¼-inch around the edges. In a bowl, mash half of pulp with a fork; add butter, milk, cheese, onion, sautéed spinach and salt. Stir well. Spoon back into skins. With the back of a spoon, create a crater in each potato. Place on baking sheet. Crack an egg into each potato half. Sprinkle with salt and pepper. Bake 35 to 40 minutes, until egg whites are set.
Sprinkle with minced rosemary and a dash or two of hot sauce.