To make the dressing, in a mason jar, combine the oil, lemon juice, mustard, garlic, chives, tarragon, and a pinch of salt and pepper. Secure tightly with lid and shake vigorously until well combined and emulsified. Give it a taste and adjust seasoning accordingly, adding more salt or pepper as needed. Set aside.
Heat a stovetop grill pan over moderate heat. Generously drizzle the bread slices with olive oil on both sides and place on the very hot grill. Cook until charred on both sides. Transfer to a cutting board and set aside to cool before cutting into cubes.
Toss the nectarine wedges in a bit of oil and add to the hot grill in a single layer. You might have to do this in batches, depending on the size of your grill pan. Cook until charred on either side, about 1-2 minutes per side. Transfer to a bowl or plate and set aside to cool slightly.
To assemble the salad, in a very large mixing bowl, combine the arugula, tomatoes, shallot, cucumber, basil, tarragon, capers, and a large pinch of salt and pepper. Add the cubed grilled bread, the grilled nectarines, and the chopped hazelnuts. Toss everything until well combined.
Transfer the mixture onto a large serving platter (or just leave in the large bowl if you'd like). Top the salad with the prosciutto pieces and small dollops of the ricotta. Serve immediately and enjoy!