A question about a recipe: The Easiest Alfredo Sauce Ever - the cheese has clumped; is there a fix? or do i have to start over?

I have a question about step 1 on the recipe "The Easiest Alfredo Sauce Ever" from lastnightsdinner. It says:

"Melt the butter in the cream in a wide, heavy-bottomed saucepan over low heat, stirring or whisking often. When the butter is completely melted, add the cheese a little at a time, whisking or stirring after each addition. Continue cooking until the sauce is reduced and thickened, about 20-25 minutes. Taste and add salt if needed. Toss with hot cooked pasta, adding a bit of the starchy pasta water to help it all come together. Serve in warmed pasta bowls, adding your choice of garnish, if desired. Pass additional grated cheese at the table."