Heat the oven to 250°C / 230°C (fan oven) / 475°F / Gas Mark 9. Line a large baking sheet with parchment. Toss the mushrooms in a large bowl with a generous amount of salt and pepper and the olive oil. Roast for 20 minutes, then toss the mushrooms once, and return to the oven. Roast for an additional 10 minutes, or until they are dark and slightly shrunken.

Return the roasted mushrooms to the bowl. Taste and add more salt and pepper if necessary. Toss with the herbs, then transfer to a serving dish and top with a dollop of sour cream. Serve immediately.