Preheat the oven to 375 F and butter a 9x13 inch baking dish.

Bring a large pot of salted water to a boil and cook the macaroni 1 minute less than the package directs, then drain.

Melt the butter in a large saucepan over medium heat, whisk in the flour, and cook for 1 minute, stirring constantly.

Gradually whisk in the milk and cook, stirring, for 5 to 6 minutes, until the sauce thickens enough to coat the back of a spoon.

Remove from heat and stir in the cheddar cheese until fully melted, then blend in the pureed spinach, salt and pepper.

Fold the drained macaroni into the sauce until every piece is coated, then pour into the prepared baking dish.

Scatter the breadcrumbs evenly over the top and bake for 20 to 22 minutes, until bubbling at the edges and golden on top.

Let rest 5 minutes, then cut small hearts from the red bell pepper and scatter them over the surface before serving.