In a mixing bowl, combine the granulated sugar, kosher salt, coarsely ground multi-color peppercorns, liquid smoke, chopped dill, and chili pepper flakes if using. Mix well until all ingredients are evenly incorporated.
Rinse the salmon fillet under cold water and pat it dry with paper towels. Place the salmon skin-side down on a large piece of plastic wrap.
Spread the curing mixture evenly over the salmon, ensuring that the entire surface is covered. Use your hands to ensure the cure adheres to the fish.
Wrap the salmon tightly in plastic wrap, pressing it down to ensure the cure is in contact with the fish. Place the wrapped salmon in a shallow dish and put a weight or heavy plate on top.
Refrigerate the salmon for 48 hours, turning it every 12 hours to ensure even curing.
After 48 hours, unwrap the salmon and rinse it under cold water to remove excess cure. Pat it dry with paper towels. Use a sharp knife to slice the salmon thinly at an angle, starting from the tail end.
Serve your homemade lox on bagels with cream cheese, on salads, or simply on its own with a sprinkle of extra dill.