Half-­fill a large saucepan with water and bring to a boil.

For round zucchini, cut off the caps and set aside. Hollow out the insides using a melon baller or spoon, leaving about a ¼-­inch (0.5 cm) shell. For baby zucchini, cut them lengthwise in half and scoop out the seeds with a small spoon to make boats. If using long zucchini, cut each one into 2-­inch (5 cm) chunks and hollow out each one from one end, leaving a ¼-­inch (0.5 cm) wall at the other end. Discard the pulp and seeds.

If using spring onions, cut them lengthwise in half, then make a small cut next to the root and scoop out the inner layers, leaving 2 outer layers. If using medium onions, cut each one into quarters and proceed as for the spring onions. Reserve the inner layers for the filling.

Cut a cap from each tomato and set aside. Hollow out the tomatoes using a spoon; reserve the pulp to add to a tomato sauce or gazpacho, if you wish. Sprinkle the insides of the tomatoes with fine salt and place them upside down on a plate lined with paper towels to drain; keep the caps next to them.

If using mini peppers, cut them lengthwise in half and discard the core and seeds. If using a large pepper, cut it lengthwise in half, then cut each half lengthwise and then crosswise in half, to make 4 boats.

Add the coarse salt to the boiling water, then add the hollowed-­out zucchini (and the caps for the round zucchini), onions, and peppers (cook these in batches if necessary). Bring the water back to a boil and cook for 4 to 5 minutes, until the vegetables are tender when pierced with the tip of knife but still bright colored (the onions should start to look translucent). Using a slotted spoon, transfer the vegetables to a bowl of ice water for 2 minutes, then remove and set upside down on paper towels to drain. Reserve 1 cup (240 ml) of the cooking water and discard the rest.

Cut the zucchini, mushrooms, and onions into 1-­inch (2.5 cm) chunks and place in a large bowl, along with the reserved inside layers of onion. Working in batches so as not to overcrowd the food processor, finely mince the vegetables, using the pulse button (some cooks use a meat grinder instead; in this case, set a colander in the bowl before adding the vegetables so any excess liquid can drain off). Transfer the minced vegetables to a large bowl.

Heat 3 tablespoons of the olive oil in a large frying pan over high heat. Add the minced vegetables, along with the thyme, and cook, stirring occasionally, until they soften, their juices evaporate, and they start to turn golden, about 10 minutes. Lower the heat to medium, add the garlic, and cook, stirring, for 2 more minutes. Season with salt and pepper to taste.

Transfer the vegetables to a bowl and stir in the meat with a fork, mixing thoroughly. Add the Parmesan or Sbrinz, eggs, basil, and parsley and season well with salt and pepper, going easy on the salt if you are using sausage meat.

To roast the stuffed vegetables

Preheat the oven to 350°F (180°C).

Rub a roasting pan with 1 tablespoon of the oil and arrange the vegetables in the pan. Stuff the vegetables generously with the meat mixture, but do not pack it down (you do not want a hard ball of stuffing). Top the tomatoes and the round zucchini with their caps. Drizzle with the remaining 1 tablespoon olive oil and pour ½ cup (120 ml) of the reserved vegetable cooking water around them.

Roast for 15 minutes, then baste the vegetables with the pan juices and roast for another 15 minutes. If the pan juices start to dry up, add a little more of the reserved cooking water. When the vegetables are browned on top, remove from the oven and baste again with the pan juices (this step is crucial).

Toss the mesclun with the vinaigrette. Serve the petits farcis warm, arranging the vegetables in a circle with the dressed mesclun in the middle.

Place the vinegar in a small bowl or jar and add the shallot or spring onion, if using, mustard, if using, and salt and pepper to taste. Whisk until combined, or seal the jar and shake it well. If you have time, you can let the shallot or onion soak in this mixture for 15 to 30 minutes to soften its flavor.

Slowly add 6 tablespoons (90 ml) olive oil (or 3 tablespoons each walnut oil and mild-­tasting olive oil), whisking to emulsify the vinaigrette, or add the oil all at once to the jar and shake until the vinaigrette looks smooth. Taste and add up to 2 tablespoons more olive oil (or 1 tablespoon each walnut and mild olive oil) if the vinaigrette tastes too acidic.