01 -
Preheat oven to 175°C. Lightly grease a 23x33 cm or a deep 23x23 cm baking dish with butter and set aside.
02 -
Heat olive oil in a medium skillet over medium heat. Add sliced onions, season with salt and pepper, and cook, stirring occasionally, until caramelized and golden brown. Remove from heat and set aside.
03 -
Using the same skillet, wipe clean, then cook the bacon until crisp. Alternatively, bake the bacon while onions cook to save time. Remove bacon, drain excess grease, and wipe skillet clean.
04 -
Melt butter over medium heat. Whisk in flour to form a roux and cook for 2 minutes. Gradually add milk while whisking continuously. Bring to a simmer and cook until slightly thickened, about 2 to 3 minutes. Remove from heat.