Preheat the oven to 350°F. Spray an 8x8-inch baking dish with non-stick cooking spray and set aside. Place the bread cubes in a large bowl. Drizzle with 1 tablespoon olive oil and toss to coat. Sprinkle with Montreal seasoning, onion powder, garlic powder, salt, and black pepper. Toss again, then spread the bread cubes in a single layer on a baking sheet. Bake for 10–15 minutes, until dry and lightly crisp.

Increase the oven heat to 375°F. Place the cut zucchini in the same bowl used for the bread. Drizzle with the remaining 1 tablespoon olive oil and toss to coat. Spread the zucchini on the baking sheet in a single layer. Roast for 20 minutes, just until tender.

In the same bowl, whisk together the cream of mushroom soup and sour cream. Add the roasted zucchini and gently stir until coated.

Spread half the toasted bread cubes in the prepared baking dish. Spoon the zucchini mixture over the bread, then top with the remaining bread cubes, pressing them down slightly so they mix with the filling.

Loosely cover the casserole with foil to prevent over-browning. Bake for 25 minutes, until the filling is hot and slightly bubbly around the edges.

Remove from the oven and let stand for 10 minutes before serving to allow the casserole to firm up slightly. Serve warm.