1. In a food processor, pulse mushrooms until finely chopped. Melt butter in a medium skillet over medium heat. Add onion; cook, stirring occasionally, until softened, about 4 minutes. Add mushrooms; cook until softened, about 3 minutes. Add port, and stir until evaporated. Stir in cream; simmer until mixture is thick, about 3 minutes. Season with salt and pepper. Remove from heat; let cool.

2. Butter a 10-by-15-inch rimmed baking sheet; set aside. Boil 1 quart water in a large saucepan; add 1 teaspoon salt. While whisking constantly, gradually add polenta. Reduce heat to a simmer; cook, whisking constantly, until soft and all water is absorbed, about 5 minutes. Stir in Parmesan. Immediately transfer to prepared baking sheet; spread evenly. Let cool completely; cover with plastic wrap, and refrigerate until ready to proceed. Polenta can be made up to 1 day ahead.

3. Halve the polenta crosswise. Carefully transfer one half, smooth side down, to a clean work surface; spread mushroom mixture on top. Place remaining polenta half, smooth side up, on top; trim edges.

4. Cut polenta lengthwise into 3/4-inch-wide rows; cut rows at an angle to form 1-inch-long diamonds. Place on parchment-lined baking sheets. Wrap in plastic wrap; freeze until ready to use, up to 6 weeks.

5. To serve, preheat oven to 425 degrees. Transfer sheets to oven. Bake diamonds, rotating sheets halfway through, 12 to 15 minutes. Garnish each with bell pepper and a dab of mascarpone, if desired. Serve warm.