Preheat oven to 350 degrees F.
In a medium saucepan, melt butter, add onion and garlic and sweat over medium heat for 3 minutes with a pinch of salt until vegetables are translucent and aromatic. Add flour and stir with wooden spoon until the flour is completely incorporated with the vegetables, cooking for about 2 minutes. Add the chicken stock 1/3 cup at a time, stirring vigorously to ensure no lumps form. Bring to boil and reduce to medium-low heat and simmer for 40 minutes. Remove pan from heat and whisk in the half-and-half, Cheddar, mascarpone, and egg. Toss sauce with macaroni and place in a 2-quart casserole pan. Mix bread crumbs and parsley in a small mixing bowl and sprinkle on top of pasta. Bake at 350 for 15 minutes, or until bread crumbs are golden brown.