Sage Walnut Pesto: Start by making the pesto. Place the sage leaves, walnuts, and crushed garlic in a food processor or high speed blender. Process on low to allow everything to be broken down into little shreds. Add the olive oil, lemon, 1/4 teaspoon of salt, and a few grinds of pepper and process again until well combined, but still rustic looking. Taste for salt and olive oil, and add more if necessary.
Sweet Potato Spanish Tortilla: Turn on the broiler. Heat a tablespoon of ghee in a large, heavy skillet over medium heat. Add sweet potatoes in an even layer. Once you've finished layering them, approximately 2 minutes later, begin stirring the potatoes so that they flip and cook evenly. Add the onions and continue to cook until both the potatoes and onions are just soft. Sprinkle generously with kosher salt and pepper. Remove the sweet potato and onion mixture to a plate.
Whisk the eggs, milk, and salt and pepper together in a medium bowl.
Meanwhile melt the remaining 1/2 tablespoon of ghee in the pan, being sure to coat the sides. Add that potato-onion mixture back in, in as even a layer as you can.
Pour the egg mixture over top, gently lifting the potatoes from around the sides of the pan to allow the egg to get into all the crevices. As the tortilla begins to turn, you want to continuously run a small spatula along the side of the tortilla, tilting the pan so that the egg can run under. Alternate this with flattening the mixture down gently with your spatula.
Once the tortilla is just about set, take the pan and place it under the broiler. Cook for approximately 2 minutes, or until set set, but not browned. Take it out and top with the optional goat cheese, if using. Place under the broiler again for another minute or two until browned, but not burned. If you're not using the goat cheese, just keep the pan under the broiler for another minute or two until browned, but not burned.
Remove from the oven and allow it to cool slightly.
You should be able to lift the whole thing right out of the pan.
Top with a sprinkling of parsley and a dollop of sage walnut pesto.