Wash the sweet potato and pierce with a pairing knife a few times. Wrap it in plastic and microwave for 8-10 minutes until tender. Check by poking it with a knife or fork, if it goes through easily, the potato is done.
Scoop out the flesh and add to a food processor or a blender with a splash of milk. Puree until smooth and set aside.
Cook pasta according to directions on the package. Add the peas into the boiling water during the last two minutes. Remove from heat, drain and set aside.
Preheat oven to 425°F.
In a medium sauce pan, melt butter over medium heat, slowly sprinkle flour over the melted butter while whisking continuously until a roux forms.
Increase heat to high and slowly stream the milk into the roux while whisking continuously. Continue to whisk the mixture until the sauce thickens.
Add the pureed sweet potato to the sauce and stir until thoroughly combined.
Turn the heat off and slowly add the cheddar cheese, whisking until the cheese is fully melted. Add salt and pepper to taste.
Mix pasta and peas with all the sauce and add to a 9”x13” baking pan.
Sprinkle Parmesan cheese on top and bake for 10 minutes.
Remove from the oven and let cool 5 minutes before digging in.