Coat a 9x13-inch baking dish with nonstick cooking spray and preheat the oven to 350°F.

Heat the olive oil in a deep skillet over medium heat. Add the zucchini and cook, stirring often, until just softened; alternatively, blanch the zucchini in boiling water for 3 to 5 minutes and drain thoroughly.

In the same skillet, brown the ground beef, breaking it up as it cooks, then drain off the excess grease. Add the diced onion and cook until translucent.

Melt the butter in a medium bowl, then stir in the seasoned stuffing mix until evenly coated. Press half of this mixture into the bottom of the prepared dish.

In a large mixing bowl, whisk together the undiluted cream of celery soup, undiluted cream of onion soup, sour cream, and shredded carrots.

Stir the drained zucchini into the soup mixture until well combined.

Add the cooked beef and onions to the bowl and stir gently. Spoon the filling over the stuffing layer, being careful not to disturb it.

Sprinkle the shredded Italian cheese blend evenly over the meat mixture.

Scatter the remaining stuffing mixture over the top, including any butter left in the bowl.

Bake uncovered for 30 minutes, until the topping is golden brown and the casserole is hot and bubbling throughout.