Heat 2 tablespoons of butter in a saute pan, add the diced red onion, and cook until soft.
Once the onion is soft and translucent, add half of the reserved zucchini pulp to the pan and cook off the excess liquid from the pulp. Transfer this mixture to a mixing bowl.
In the same pan, brown the ground beef, then add to the onion and zucchini mixture the mixing bowl.
Add the bechamel to the mixture and stir to combine. Add freshly grated parmesan cheese, salt, and pepper to taste.
Arrange the zucchini halves in a baking dish, cut side up. Season lightly with salt and pepper, then spoon a generous amount of filling into each half.
In a small bowl, toss about a cup of panko bread crumbs with a drizzle of olive oil, just enough to coat. Top each zucchini half with some of the bread crumb mixture.
Bake at 350 degrees until the zucchini is soft, the filling is warmed through, and the bread crumbs are golden.