Place goat cheese, scallions, mint, red-pepper flakes, and reserved piquillo liquid in a bowl. Zest and then juice lemons into the bowl, straining seeds. Mix until well combined.
Using a small spoon, fill each pepper with about 2 tablespoons of the goat cheese mixture. Alternatively, pipe the mixture into the peppers. Place on a serving plate, and sprinkle with red-pepper flakes; garnish with spring onions if desired. Serve immediately, or cover and refrigerate until ready to serve, about 1 hour.
Piquillos are small, sweet fire-roasted red peppers that are packaged only in Spain. Jars of piquillos are available online and at specialty-food stores.