1. Trim the ends off the zucchini and if small, slice them lengthwise. If they are larger, cut them into batons about 3/4 inch (2cm) thick. Place them in a colander and toss them with 2 teaspoons of salt. Let drain for 1 to 2 hours, shaking and turning them a few times, coaxing the excess water to drain away.
2. Heat the olive oil in a skillet over medium heat. Add the scallions, thyme, some freshly ground pepper, the chili flakes, and 1/2 teaspoon salt. Cook, stirring frequently, until the scallions are wilted and soft, about 3 minutes. Remove from heat and set aside.
3. Preheat the oven to 450ºF (230ºC). Blot the zucchini dry with a paper towel and toss them with a bit of olive oil on a baking sheet. (You can line the baking sheet with parchment paper for easier clean up if you want, or if you’re parsimonious with the olive oil and are concerned they’ll stick.) Spread them in an even layer, cut side down, and roast them in the oven just until tender, about 15 minutes, but don’t cook them to mush.
4. Remove from the oven and place the zucchini in a single layer in a shallow baking dish or gratin dish that will fit them snugly in an even layer. Strew the scallions over the zucchini. Drain the tuna and flake it over the scallions and sprinkle the cheese evenly over the top.
5. Bake until the cheese is melted and starts to brown and the casserole is heated through, 10 to 15 minutes. Serve warm.