Remove zest of oranges in strips with a vegetable peeler and cut away any white pith from strips. Squeeze juice from oranges and lemon into a 6-quart pot and add lemon halves, water, granulated sugar, and zest. Peel pears, leaving stems attached, and add to pot.
Simmer pears 15 to 25 minutes, or until just tender, and cool in liquid. (Poached pears will continue to cook a bit as they cool.)
Put granulated sugar in a 2-quart heavy saucepan and crumble brown sugar over. Melt sugars, undisturbed, over moderate heat until granulated sugar is mostly melted. Continue to cook, stirring occasionally with a fork, until a deep golden caramel. Carefully pour in cream (mixture will bubble up and caramel will harden slightly) and stir in clove, cardamom pods, fennel, peppercorns, and cinnamon sticks. Simmer, stirring, until caramel is dissolved and sauce is reduced to about 1‚ cups, about 10 minutes.
Pour sauce through a sieve into a 2-cup measure and cool to just warm.
Drain pears and pat dry, then transfer to plates.
Serve drizzled with sauce and sprinkled with chopped nuts.
• You can poach pears 2 days ahead and chill, covered, in poaching liquid. Bring pears to room temperature before serving. • Sauce may be made 2 days ahead and chilled, covered. Reheat to warm before serving.
