Prepare the filling: Heat 1 tablespoon vegetable oil in a wok over medium-high heat. Add minced garlic and sauté for 30 seconds until fragrant. Add ground pork and cook for 5-6 minutes, breaking it apart with a wooden spoon until browned. Add chopped mushrooms, 1 tablespoon fish sauce, black pepper, and continue cooking for 3 minutes. Remove from heat and let cool.

Make the batter: In a large mixing bowl, whisk together rice flour, water, and a pinch of salt until completely smooth with no lumps. The batter should have a thin, watery consistency similar to milk. Let rest for 10 minutes.

Prepare the steaming setup: Set up a steamer with boiling water. Lightly oil a flat steaming pan or plate (8-10 inches diameter). Keep a bowl of cold water nearby with a pastry brush.

Steam the rice sheets: Stir the batter before each use. Ladle about 1/3 cup of batter onto the oiled steaming surface, tilting to spread evenly into a thin layer. Cover and steam for 1-2 minutes until the sheet becomes translucent and set. Use a spatula to carefully lift the edges.

Fill and roll: Place 2 tablespoons of the pork-mushroom filling in the center of the steamed sheet while still on the steaming surface. Fold the bottom edge over the filling, fold in the sides, then roll up completely. Transfer to a serving plate. Repeat with remaining batter and filling.

Prepare the dipping sauce: In a small bowl, combine fish sauce, lime juice, minced garlic, sugar, and water. Stir until sugar dissolves. Add sliced chili if desired.

Serve: Arrange banh cuon on plates, garnish generously with fresh cilantro, mint leaves, and crispy fried shallots. Serve immediately with dipping sauce on the side.