Cook pasta according to package directions. As soon as the pasta goes into the water, begin making the sauce.
Heat large, high sided skillet over medium heat.
Add to butter to skillet and let melt, stirring occasionally. Continue stirring as the butter begins to fizzle and bubble as the water content boils away
When milk fat particles in the butter begin to take on color, stir in chopped sage leaves, salt, and pepper.
Whisk in heavy cream until well combined. When the mixture is cohesive with no separation, add the pasta directly from the pasta pot and stir to coat.(And if the sauce separates, don't worry — it'll come together when you add the pasta coated in starchy pasta water!) Stir in half the apples and walnuts, and cook for 1-2 minutes, stirring occasionally to soften and brown the apples. Divide into bowls and serve immediately. Top with remaining minced apple, chopped walnuts, and additional chopped sage if desired.