Polenta, which is boiled cornmeal, becomes elegant and distinctive in these appetizers.
1. Make filling: Melt butter in skillet, add onion, and sauté until translucent, about 2 minutes. Add mushrooms and continue cooking until softened, about 3 minutes. Transfer to a food processor and finely chop. Return mixture to skillet, add heavy cream and simmer until mixture thickens, about 5 minutes. Season with salt and pepper and set aside.
2. Prepare polenta: Bring chicken broth to a boil. Add polenta and stir constantly until broth is absorbed. Stir in both Parmesan and ½ cup blue cheese. Immediately spread evenly over the foil-lined baking sheet. Set aside to set, about 30 minutes.
3. Mix the remaining 2 tablespoons of blue cheese with cream cheese until smooth. Transfer to a sandwich bag. Set aside.
4. Make diamonds: Cut polenta in half. Transfer one piece to a cutting board and completely cover it with mushroom filling, spreading it evenly. Top with remaining piece of polenta. If using later, wrap tightly with plastic wrap and refrigerate for up to one week. Otherwise, cut polenta on an angle into six strips 3/4-inch wide and 1 inch long, creating diamond shapes.
5. Just before serving, snip the corner of the sandwich bag holding the creamy blue cheese mixture and use as a pastry bag to put a curl of the mixture on top of each diamond. Decorate with chervil and thin slices of red bell peppers if desired.