1Preheat oven to 500 degrees with a rack set in upper third of oven.
2Place trout on parchment place baking sheet skin-side down. Season cavity of each trout generously with Provence evoo, salt and pepper; top each with 2 to 4 sprigs of thyme and 3 lemon slices. Fold over to enclose the trout and season the skin with salt and pepper; brush lightly with more Provence evoo. Transfer to oven; cook 4 to 5 minutes. Rotate pan and continue cooking until skin is crispy and golden brown and flesh is just opaque throughout, 4 to 5 minutes more.
3Meanwhile, in a large bowl, toss watercress, radishes, feta, and walnuts with vinaigrette. Divide salad evenly between 4 plates. Transfer trout to plates and serve immediately with a dollop of creme fraiche.