For salad: Preheat oven to 400 degrees. Toast walnuts about 5 minutes. Place pears, cut side up, in glass or ceramic baking dish with fennel. Drizzle with olive oil and vegetable stock. Cook in oven for 10 minutes. Turn pears and fennel over and roast until pears are tender, approximately 5 minutes. Transfer pears to plate. Continue cooking fennel until tender, approximately 10–15 minutes. Transfer fennel to plate with pears. When cool, thinly slice pears and fennel lengthwise.

For vinaigrette: Reduce port and shallots in a heavy saucepan over medium heat to ¼ cup. Strain into a large bowl and cool. Whisk in oil, vinegar, salt, and pepper until thoroughly combined.

Toss the mâche, pears, fennel, and walnuts in vinaigrette. Serve and garnish with extra pears and walnuts.