Melt oil and butter in a large skillet over medium heat. Add sliced onions, stir and cook slowly, stirring occasionally for 20-30 minutes or until completely soft and tender. If your onions are coloring before they are tenderizing, your heat may be too high. Reduce heat and continue cooking.
Place heavy cream, rosemary sprigs, and sage leaves In a small saucepan over medium-low heat. Cover and simmer while you prepare the remainder of the ingredients.
Bring a large pot of water to a boil, salt generously and add pasta and cook to package directions for al dente, stirring occasionally. Reserve some of the pasta water, drain pasta, and set aside.
Using a sharp knife (or corn on the cob peeler), cut the corn off the cob. Once kernels are removed, run the back of your knife down the husk to remove the bottom parts of the corn. Set aside.
Once onions are almost caramelized, add diced zucchini and yellow squash. Season with salt and black pepper. Stir occasionally and cook until squash is semi-tender (personal preference).
Once the onions are fully caramelized and the squash is cooked to your liking, reduce the heat to low and add the heavy cream to the skillet (straining out the herbs). Discard herbs.
Stir in parmesan cheese. Add corn to the sauce to warm through. Add the pasta to the skillet and coat with sauce, adding the reserved pasta water as needed for your desired consistency.
Garnish with fresh chopped basil and serve warm.