Place dried porcini in a medium bowl, if using, and pour 2 cups of boiling water over them. Soak until softened, at least 20 minutes.
Meanwhile, bring a large pot of water to a boil. Heat the oven to 425F. Butter a 9 1/2-inch spring-form pan. Tightly wrap the bottom of the pan with a large sheet of foil, crimping the foil against the outer edges to tightly seal
In a large skillet, heat 1 tablespoon of oil over medium-high heat. Add the onion and garlic and cook for 1 minute. Add the mushrooms, season with salt and pepper and cook, stirring occasionally, until browned, 10 to 12 minutes. Scrape into a bowl. Drain the porcini mushrooms (discard liquid or save for another use). Coarsely chop reconstituted porcini and add them to the cooked mushroom mixture.
In the same skillet, heat the remaining 1 tablespoon of olive oil. Add the kale, season with salt and pepper and cook over moderate heat, stirring, until just wilted, about 2 minutes. Fold kale into mushroom mixture and set aside.
Add salt to the boiling water and cook the spaghetti just until very al dente, 7 to 8 minutes (look for a white spot in the center when you bite into a strand). Drain the pasta (do not rinse) and reserve the pot.
In the pasta pot, whisk together the milk, eggs, pepper, and 3⁄4 teaspoon salt. Stir in the cheddar, fontina, and 1 cup of the Parmesan. Add the Rao’s Homemade Tomato Basil Sauce, mushroom and kale mixture, and the Spaghetti; stir until well combined.
Transfer the pasta mixture to the prepared pan. Using a spatula, smooth the top. Set the pan on a rimmed baking sheet and bake until the edges are golden and bubbling, about 35 minutes.
Remove the pan from the oven. Turn on the broiler. Sprinkle the pie with the sage and the remaining 1/2 cup of parmesan. Broil 4 to 5 inches from the heat until the cheese is golden, 2 to 3 minutes. Remove from the oven and run a knife around the inside of the pan. Let the pie rest about 10 minutes then release and remove the sides of the pan. Cut the pie into slices and serve warm.
Save the porcini liquid to add to soups and stews along with stock to a ton of added flavor.
Fresh herbs folded into the mixture can be a great flavor boost too. Use up any leftover basil, parsley or oregano you may have in the fridge instead of, or in addition to, sage. Don’t be shy to fold a handful into the spaghetti mixture before baking