Cut each zucchini crosswise into 3 pieces and hollow out the insides with a melon baller or small spoon. Finely chop the zucchini flesh and set aside in a bowl.

Put the ground beef, cilantro, tomatoes, chopped onion, rice, 1½ teaspoons salt, ¼ teaspoon pepper, and the water in a large bowl. Mix the filling ingredients with your hands or a wooden spoon until well combined. If you want to check the seasoning, fry a small spoonful of the filling and taste; adjust with more salt and pepper if needed.

Stuff the vegetables: Stuff the zucchini pieces and the bell peppers with the filling (the center pieces of the zucchini will be open on both ends, so just pack the filling tightly). One at a time, lay the grape leaves flat on the work surface and spoon about 2 tablespoons of filling on to the middle of the lower half of the leaf. Fold the sides over the filling and roll up the leaf from the bottom, tucking in the left and right sides as you go; the result should look like a small burrito or spring roll. 

Heat the olive oil in a Dutch oven or other large heavy-bottomed pot over medium heat. Add the sliced onion and cook until nearly translucent, 8 to 10 minutes; don't let the onion brown. Add the reserved chopped zucchini flesh, the passata, and 1 teaspoon salt, bring to a simmer, and cook, stirring constantly, until the sauce is slightly reduced and thickened, 2 to 4 minutes. Taste and adjust the seasoning with more salt if needed.

Reduce the heat to low and carefully layer in the stuffed vegetables, tucking the grape leaves around the zucchini and peppers. Stir 1 teaspoon salt into 1/4 cup water and pour it over the vegetables. Place an upside-down plate on top of the stuffed vegetables; alternatively, cut a round of parchment paper to fit and lay it over the vegetables.

Cover the pot and simmer the stuffed vegetables until completely tender, 1 to 1½ hours. Carefully transfer the vegetables and grape leaves to a large platter. 

Taste the sauce and adjust the seasoning with more salt and pepper if necessary, then spoon it around the vegetables. To serve, carefully cut the peppers in half, and give everyone at least one portion of zucchini, pepper, and stuffed grape leaf, along with some of the sauce.