Slice polenta in half. Slice each half into six rounds, about 1/2 inch thick, so you have 12 rounds total.
Heat oil in large nonstick skillet over medium-high heat. Gently slide each polenta round into hot oil using spatula. Fry for 5 minutes per side or until golden brown and cooked through. Remove from oil and drain on paper towel-lined sheet tray.
In medium bowl, combine beans, salsa and 2 teaspoons taco sauce. Mash beans with fork.
In small bowl, combine sour cream, remaining taco sauce and lime juice, and mix until well blended.
Spread 2 tablespoons mashed black beans on top of a fried polenta round. Top with a pinch of shredded lettuce and about 1 tablespoon salsa, and drizzle with some sour cream mixture.