In a large pot of boiling salted water, cook the pasta until it’s tender but not mushy. Drain well in a colander and set aside.

Preheat the oven to 350°F.

In the empty pot (no need to wash it out), melt the butter over medium heat. Add the flour and stir well with a whisk, cooking for a minute or so until the mixture starts to turn golden. Stir in the milk and bring the sauce to a boil, whisking constantly. The sauce must reach a full boil in order for the flour to reach its full thickening potential. Reduce the heat and simmer for a few minutes, until the mixture is nice and thick.

Remove the sauce from the heat and stir in the cheese until it melts. Stir in the vegetable puree, if you’re using it. Add salt to taste, then stir in the drained pasta. If you want a bread crumb topping, pulse the bread, butter and Parmesan in a food processor until the bread turns to crumbs and the mixture is well blended. Pour the macaroni and cheese into an appropriately sized baking dish and top with the bread crumbs or additional cheese. (It can be made up to 24 hours ahead and refrigerated in the baking dish; sprinkle with the crumb mixture or cheese right before you bake it.) Bake for 15-20 minutes, until the topping is golden and it’s bubbly around the edges.