Preheat the oven to 375℉.
Cook the noodles according to package directions. Drain and set aside.
Meanwhile, in a pretty big pot (like a Dutch oven) over medium heat, heat the first 1/4 cup of butter with the onions and the herb. When the butter's bubbly, stir in the flour with the whisk. Add the milk, stirring till it thickens slightly.
Use the spoon to stir the tomatoes and the cheeses into the sauce. It should be thickish, and if the heat's not too high, it won't curdle. If it does, take it off the stove, whisk the bejeezlies out of it and use it anyway!
Pour the pasta into a serving dish that is bigger than you think you need, cos you need tossing room. Pour the sauce over it and fold it in.
Sprinkle cereal crumbs all over the top, covering completely. Pour the butter over evenly as possible.
Bake about 25 minutes. Let sit up to ½ hour before serving!
