Place a cast-iron enameled Dutch oven over high heat and add the olive oil. Add the onions, garlic and butter and briefly cook until the onion softens, 2 to 3 minutes, then stir in the chili flakes. Add the mussels, Chardonnay and chicken stock. Season with salt and pepper. Cover and cook until all of the mussels have opened, 5 to 7 minutes. Remove the lid then add the parsley.
Preheat a grill pan over medium heat. Brush the bread slices with olive oil on both sides. Grill until toasted on both sides, 30 seconds per side.
Serve the mussels with a rustic piece of grilled bread.