Finely dice onions. Finely chop parsley. Heat olive oil in a pan. Add onions, cooking on medium heat for 5 minutes. Reduce heat to low and cook for an additional 30 minutes, or until onions have browned. The heat should be hot enough that the onions barely sizzle and are not cooking aggressively.
While onions are cooking, prepare macaroni. Boil macaroni in salted water and drain. Set aside.
Next, prepare cheddar sauce. Shred cheese and set aside. Melt butter in a pan on medium heat. Stir in flour, and add salt and pepper to taste. Add parsley and gradually add milk, stirring constantly. Keep stirring occasionally until sauce thickens. Stir in cheese.
Mix cheese mixture together with onions and macaroni.