Begin by heating a small knob of butter with a dash of olive oil in a pan over medium-high heat. Add the bacon and sauté gently for 3-5 minutes, or until it becomes lightly golden and crispy. Once done, transfer the bacon to a plate lined with kitchen paper and give the pan a light wipe.
Next, add a larger knob of butter along with a splash of oil to the same pan. Add the sliced onion and and gently fry for 10–15 minutes until it becomes soft, caramelized and sweet. Remove from pan and set aside.
Add the remaining butter along with the Brussels sprouts, seasoning generously with salt and pepper. Continue cooking, tossing occasionally, for about 5 minutes, or until the sprouts are crisp, slightly tender and lightly browned. Return the crispy bacon and caramelized onion to the pan, add the parsley, and mix thoroughly to ensure all the ingredients are well combined.
In a medium mixing bowl, whisk together the eggs, cream, cheeses and season with salt and freshly ground pepper. Pour the egg mixture over the Brussels sprouts and bacon in the pan, making sure everything is evenly coated.
Allow the mixture to cook for about one minute, then cover the pan with a lid. If you don’t have a lid handy, you can use a layer of baking parchment followed by tinfoil. Transfer the pan to the oven and bake for 30-40 minutes, or until the frittata is set. For the final few minutes of cooking, remove the lid or parchment/foil to achieve a lovely golden finish.
Serve with winter greens or vegetables for a delicious brunch or lunch.