Bring salted water to a boil in a large pot. Add the pasta and cook according to the package directions.
Meanwhile, in a medium bowl, combine the pear and lemon juice, and toss to coat the pear evenly to prevent them from turning brown.
In a sautué pan over medium-high heat, melt 2 tablespoons of the butter. Add the pears and sugar and caramelize. Set pears aside on a plate.
Add the remaining butter and onions and saute until tender. Add the cream and reduce the heat to low and simmer for about 1 minute. Add ½ a cup of the cheese and whisk until smooth, about 1 minute more. Remove from the heat and season with salt and pepper.
Add the pears, walnuts, arugula and basil and stir. Drain the pasta in a colander, add to the sauce and toss well. Add the remaining cheese and serve immediately.