Preheat your oven to 500 °F. Cut each zucchini in half lengthwise and scoop out the flesh, leaving a firm border to hold the filling. Brush with olive oil, season with salt, and roast for 10 minutes.
Heat olive oil in a saucepan and sauté the zucchini flesh until soft. Add garlic, then spinach in batches, seasoning each batch lightly. Remove from heat, mix in dill, cheeses, black pepper, and red pepper flakes.
In another pan, sauté onions in olive oil until golden. Add garlic, then pureed tomatoes, sugar, salt, pepper, and oregano. Simmer until slightly thickened, tasting and adjusting seasoning.
Lower oven temperature to 400 °F. Pour tomato sauce into a baking dish, place zucchini boats on top, and fill them with the stuffing. Sprinkle pine nuts and chopped olives over the top.
Bake for 25–30 minutes until the zucchini is tender and the filling is golden. Let cool slightly before serving for best texture and flavor.