In a large skillet or saucepan over medium/high heat, melt the butter and add the flour. Whisk until smooth and cook for an additional 30 seconds. Slowly pour in the milk while whisking. Add in the brown sugar, salt, pepper, and nutmeg. Continue to cook stirring frequently until the mixture thickens, about 5 minutes. Add in the shredded cheese, stir to combine. Add in the peas and pasta and mix together. Pour in reserved pasta water if the sauce seems too thick.