Cut both ends off the zucchini, then split them in half lengthwise. Using a small measuring spoon or teaspoon, carve out a hollow in each zucchini half so they look like little 'canoes'. (In Italian they call the barchette or 'little boats', but to me canoe is more like it.) Chop the pulp and reserve.

Place the zucchini canoes in a steamer, sprinkle with salt, and steam them until they are crisp-tender.

While the zucchini are steaming, sauté the onion in olive oil until soft and translucent. Add the reserved zucchini pulp and a pinch of salt and pepper. Let the zucchini pulp cook for a few minutes to absorb the flavors of the oil and onion and lose its excess liquid. Allow this mixture to cool.

Mix the zucchini mixture with the rest of the stuffing ingredients in a large bowl until you have a homogenous mixture. Taste the mixture and adjust for seasoning; it should be very tasty. When the zucchini are done, remove them from the steamer and let them cool upside so they can drain, on a rack or kitchen towel.

When the zucchini are cool, stuff their cavities and lay them out snugly in a baking dish. Pour over a glassful of water or white wine (mixed with the tomato paste, if using). Drizzle with olive oil. (You can make the recipe ahead up to this point.)

Bake the zucchini in a hot oven (200°C/400°F) for about 20 minutes or so, until the stuffing is cooked through and the tops nicely browned. (You can test doneness with a paring knife or skewer, if the stuffing has a firm texture and the knife or skewer comes out clean, it's done.) Let the zucchini cool, at least a bit, before serving. They are fine warm or at room temperature, but not scalding hot.