You can substitute any slightly sweet white wine for late-harvest riesling. Heat the pound cake on a grill pan before serving to create attractive grill marks. Garnish with raw pear slices.
Combine first 6 ingredients in a large saucepan; bring to a boil. Cover, reduce heat, and simmer 15 minutes. Stir in tea leaves; simmer for 2 minutes. Drain wine mixture through a fine sieve into a bowl; discard solids.
Return wine mixture to pan; add pear halves, cut sides down. Cover; simmer 15 minutes or until tender. Remove pears with a slotted spoon; place in a shallow dish. Set pears aside; keep warm.
Bring wine mixture to a boil over medium-high heat; cook until reduced to 1 cup (about 10 minutes). Add mascarpone, stirring with a whisk until well blended. Remove from heat; add butter, stirring until butter melts.
Place 1 pound cake slice on each of 8 plates, and top each slice with 1 pear half. Spoon about 2 tablespoons sauce over each serving. Serve warm.