Heat oven to 200C/180C fan/gas 6. Brush the bread with 1 tbsp of the olive oil and scatter over some sea salt and thyme leaves. Roast for 15 mins, turning once, until crispy and golden. Break up into a bowl and toss with the capers.
Meanwhile, whisk together the lemon juice, 2 tbsp olive oil, a pinch of salt and the mustard to make a dressing, then pour half over the bread.
Heat a grill pan to a high heat and sprinkle in some sea salt. Drizzle the nectarines with the remaining 1 tbsp oil and grill, flesh-side down, until charred. Add to the bowl with the bread, along with the basil, sorrel, chervil and tarragon.
Season and toss everything together, then pile onto a serving platter. Top with the ricotta, prosciutto and hazelnuts, and serve with the remaining dressing on the side, if you like.