Caramelize the Onions
Melt 40g of butter in a skillet over medium-low heat. Add all the finely diced onions and a pinch of salt. Cook slowly, stirring occasionally, until they turn a deep, uniform golden brown. Divide the finished onions in half: one half goes into the filling, the other half is saved for the final glaze.
Slow caramelization coaxes out natural sugars, providing a sweet contrast to the sharp cheese and peppery filling.
Soft, rich, amber-colored onions. Do not let them char or crisp up.