Cook the rice according to the package instructions.

While the rice cooks, cut the zucchini in half lengthwise (no need to trim the ends!). Using a small spoon, scoop out the flesh of the zucchini, leaving a ¼-inch-thick shell to form zucchini boats. Season the zucchini boats on all sides with kosher salt. Arrange them on a baking sheet and set them aside.

Finely chop the zucchini flesh to yield 1 cup (discard the remaining flesh). Peel and finely chop the onion. Peel and mince the garlic. Trim and finely chop the scallions.

Preheat a smoker to medium-high heat (about 400ºF).

While the smoker preheats, heat the olive oil in a large skillet over medium-high heat. Add the onion, zucchini flesh, and a pinch of salt. Cook, stirring occasionally, until the onion is softened, about 5 minutes. Add the ground beef and garlic. Season with salt and cook, using a spatula to finely break up the meat, until the beef is browned, 8-10 minutes. Add the tomato sauce, cooked rice, and spinach. Cook, stirring, until the spinach is wilted and the mixture is bubbly and thickened, 10 minutes more.

Fill the zucchini boats with the beef mixture. Top them with the cheeses and chopped scallions.

Add the zucchini boats directly to the preheated smoker and cook until the cheese is browned and the boats are tender, 20-25 minutes