Cook the fettucine according to the package. Drain in colander. Reserve 1/4 cup of pasta cooking liquid.
While pasta is cooking, melt butter in medium saucepan over medium high heat. Add onion/shallots and saute until tender. Add heavy cream and bring to boil. Cook until sauce has reduced slightly, about 5 minutes. Remove from heat.
Return pasta to pot it was cooked in, set over medium high heat along with the reserved cooking liquid. Add butter cream mixture and half of parmesan cheese. Toss to combine thoroughly. Season with salt and pepper to taste. Sprinkle with remaining Parmesan and garnish with parsely, if desired. Serve immediately.