Soak cashews in water for 4 hours or overnight. The water should be 1 inch above the cashews.
Preheat oven to 350 and grease a baking sheet. Cut the pumpkin in half and remove seeds and strings (if you have a pumpkin carving tool, the scooper works great for this!). Brush the cut side of the pumpkin with olive oil Prick the skin side of the pumpkin with a fork a few times then place the pumpkin cut side down on the baking sheet and start baking.
Toss the onion and garlic cloves in olive oil. After pumpkin has been baking for 10 minutes add the onion and garlic to the sheet pan and continue to bake for another 25-35 minutes or until the pumpkin starts to deflate and is soft.
In the last 10 minutes of baking, start boiling water for the pasta and cook pasta according to the directions on the bag. Set a time for its cooking time so you can work on the sauce while it's cooking.
Drain and rinse the cashews. One pumpkin is done cooking and has cooled enough to handle, use a big spoon or the scoop from a pumpkin carving tools to scoop out the pumpkin from the peel. Remove the roasted garlic from its paper-like peel.
In a high powered blender, combine onion, garlic, cashews, pumpkin, broth and seasonings. Puree until velvety smooth.
Toss 1/3 of the sauce with hot pasta (continue heating as needed) and serve with a few fried sage leaves.
I've found this makes about 3 dinners worth of alfredo pasta sauce for a family of 4 so I use one the day I make it then freeze the other 2 portions in freezer safe plastic bags.
You may also use canned pureed pumpkin. Use one full can for the recipe. The pasta will be more orange in color than if you were using pureed roasted pumpkin.