To make the homemade tomato sauce: Heat the oil in a heavy large saucepan over medium-high heat. Add the onions and sauté for about 5 minutes or until the onions soften. Add the garlic and sauté for one minute. Stir in the white wine.
Add the fresh and canned tomatoes, thyme and bay leaf. Bring to a gentle simmer then decrease the heat to medium-low so that the sauce simmers gently. Allow the sauce to gently simmer for about 1 ½ hours or until the tomatoes are mushy and the sauce thickens slightly, stirring occasionally and adding a little water if necessary to cover the tomatoes before they become tender.
Remove the sauce from the heat and allow to cool. Place the tomato sauce in a 15x10-inch/38x25-cm baking dish.
To prepare the sherry glaze and zucchini: Boil the sherry vinegar in a heavy small saucepan over medium-high heat until reduced to a glaze, about 18 minutes. Remove from the heat and allow the sherry glaze to cool. Using a teaspoon, scoop out the seeds from each zucchini half.
Stir the ricotta, mozzarella and Parmesan cheeses in a bowl to blend. Season the cheese mixture to taste with pepper.Stuff the zucchini with the cheese mixture, covering the cut side of the zucchini halves and mounding slightly.
Place the zucchini halves atop the tomato sauce in the baking dish. Bake in the oven for about 10 minutes, or until the zucchini are tender and the cheese filling is heated through.
Preheat the broiler. Transfer the baking dish to the broiler and broil for 30 seconds, or until the cheese filling is golden on top. Place the stuffed zucchini on plates with a little tomato sauce. Drizzle the sherry glaze around the outside edge of the plates and serve.