1. Preheat the oven to the high broiler setting and place one of the oven racks in the second-highest position. Line a large baking tray with parchment paper.
2. Clean the mussels under cold, running water. Remove any hair connected to the mussels and discard any mussels that are completely or even slightly open. Set aside.
3. Fill a large saucepan with 1 cup (250 mL) of cold water, cover with a lid, and bring to a boil over high heat. Add the mussels, cover, and steam for 2 to 3 minutes, or until all the mussels have opened. Discard any mussels that do not open.
4. Once the mussels have cooled, grip the top shell in one hand and the bottom shell in the other hand, and give a small pull to detach only the top shell. Repeat with the remaining mussels. Discard the top shell of each mussel, leaving just the meat in the bottom half of the shell. Place the mussels on the prepared baking tray.
5. In a large mixing bowl, combine the cheese, sour cream, heavy cream, and chili flakes.
6. Spoon 1 tsp (5 mL) of the cheese mixture into each half mussel, using the back of a separate spoon to pack it in tightly.
7. Place the mussels under the broiler for 3 to 5 minutes, or until the cheese is bubbly and golden brown. Watch closely so the cheese doesn’t burn.
8. Remove the mussels from the broiler, transfer them to a serving platter, and garnish with diced chives.