Heat oven to 350 degrees F.
In 10-inch nonstick, ovenproof skillet, heat the butter over medium-high heat. Add the onion; cook, stirring occasionally, until softened, about 5 minutes. Add the sliced potatoes; cook, stirring occasionally, for 5 minutes. Stir in spinach; cook, stirring occasionally, until the spinach is wilted, about 3 minutes.
In a medium-size bowl, whisk together the eggs, milk, salt, ground nutmeg and cayenne until well blended. Stir in 1 cup of the shredded Jarlsberg cheese and the sliced pimiento.
Pour the egg mixture over the potato-spinach mixture in the skillet; cook over medium-low heat until eggs are barely set, about 5 minutes, gently stirring the middle of the mixture about halfway through the cooking. Sprinkle the remaining 1/2 cup shredded Jarlsberg cheese evenly over the top of the frittata.
Place the skillet in the 350 degree F oven and bake, uncovered, until the frittata is golden brown on top and the egg mixture is completely set in the center, about 10 minutes.
To serve, gently slide the frittata onto a serving platter. Cut into 4 equal wedges. Serve the frittata warm or at room temperature. Makes 4 servings.