Set your oven to 375 °F (190 °C).
In a small skillet over medium‑high heat, melt the butter. Add the finely chopped mushroom stems and shallots, and cook for 4–5 minutes until soft. Stir in the garlic and chopped walnuts, season with a pinch of salt, and cook for 2 more minutes.
Remove the pan from the heat. Stir in the parsley, herbes de Provence (or thyme), and breadcrumbs. Transfer everything to a food processor and pulse until you have a coarse paste.
Toss the mushroom caps in olive oil and arrange them gill‑side up on a baking sheet.
Spoon the walnut‑garlic mixture into each cap. Sprinkle the grated Parmesan evenly over the tops.
Bake for 10–20 minutes, until the cheese turns lightly golden and you see a little moisture at the base of each mushroom. Let them rest for about 5 minutes before serving.