Preheat oven to 375 °F (190 °C). Slice the tops off the tomatoes, zucchini, and peppers, reserving the lids. Halve the onions crosswise and hollow out all vegetables, leaving a 1 cm wall. Reserve the pulp.

Chop the reserved vegetable pulp finely and sauté in olive oil over medium heat for 8 minutes until softened. Season with salt and pepper, then let cool slightly.

Squeeze excess milk from the soaked bread and tear into a large bowl. Add the ground meat, sautéed vegetables, eggs, garlic, parsley, basil, thyme, and Parmesan. Mix by hand until well combined but not dense. Adjust seasoning.

Drizzle a shallow baking dish with olive oil. Arrange the hollowed vegetables upright, packed snugly so they support each other. Fill each vegetable with the stuffing, mounding slightly above the rim. Replace the lids at an angle.

Drizzle the tops with olive oil and season with fleur de sel. Roast for 45–55 minutes until vegetables are tender, golden, and the cheese is browned. Juices should bubble at the bottom.

Remove from the oven and let rest for 10 minutes. Garnish with fresh basil and a drizzle of cold-pressed olive oil. Serve with crusty sourdough and tomato dipping sauce if desired.