Step 1Cook farro in plenty of rapidly boiling salted water until tender, about 30 minutes. Drain and cool.
Step 2Heat the oven to 400 degrees. Grease a 5-quart gratin dish with olive oil.
Step 3Cut the zucchini in half lengthwise and use a melon baller or serrated spoon to remove the insides of the zucchini to make what looks like a canoe. If using a melon baller, use the large scoop to remove most of the pulp in balls and then the small scoop to smooth the sides. Leave about one-quarter inch of the flesh at the sides and a little more at the bottom. Collect the pulp on a cutting board and chop coarsely.
Step 4Season the inside of the zucchini boats lightly with salt and steam over rapidly boiling water until almost tender, about 5 minutes.
Step 5Heat 2 tablespoons olive oil in a large skillet and add the onion. Cook until it softens, about 5 minutes. Add the red bell pepper and cook until soft, about 5 minutes. Add the chopped zucchini pulp and basil and cook until dry, about 10 minutes.
Step 6Increase the heat to high. Add the mushrooms and cook until soft, 5 minutes. Add the garlic and red pepper flakes and cook until fragrant, about 3 minutes. Add the white wine and cook until dry. Set aside to cool.
Step 7Stir the cooked farro, pine nuts and feta into the cooled vegetable mixture. Season to taste with salt and freshly ground black pepper, and spoon the mixture into the hollowed-out zucchinis, mounding on top. It will take 4 to 6 tablespoons per zucchini half.
Step 8Arrange the stuffed zucchini in the gratin dish; they may fit quite tightly. Sprinkle the tops with more crumbled feta. Pour the water into the baking dish so it just covers the bottom. Bake until the tops have browned, about 15 to 20 minutes.
Step 9Drizzle the top of each zucchini with a little more olive oil, transfer to a serving platter and scatter over more slivered basil. Serve warm or at room temperature.