Baked Macaroni and Cheese

kosher salt
vegetable oil
1 lb. elbow macaroni
4 c 2% milk
6 T unsalted butter
1/2 c flour
8 oz. sharp cheddar
8 oz medium cheddar
8 oz colby jack
1/2 t black pepper
1/2 t nutmeg

topping:
bread crumbs
olive oil
thyme
rosemary
2 can diced tomatoes