Begin by preparing the caramelized onions. In a large, heavy-bottomed skillet or Dutch oven, melt 30g (2 tbsp) of unsalted butter with 1 tbsp (15ml) of olive oil over medium-low heat. Add the thinly sliced yellow onions.

Cook the onions slowly, stirring occasionally, for 30-45 minutes, or until they are deeply golden brown, soft, and sweet. This slow cooking process is crucial for developing their rich flavor. Be patient; rushing this step will result in undercooked, less sweet onions.

Once the onions are beautifully caramelized, remove them from the skillet and set aside in a bowl. Wipe out the skillet if necessary, but don't wash it thoroughly, as the residual flavor can be beneficial.

While the onions are caramelizing, prepare the potatoes. Place the peeled and cut Russet potatoes in a large pot and cover them with cold water by at least 2.5cm (1 inch). Add a generous pinch of salt (about 1 tsp or 5g) to the water.

Bring the water to a rolling boil over high heat, then reduce the heat to medium and simmer for 15-20 minutes, or until the potatoes are fork-tender. They should be easily pierced with a fork with no resistance.

Drain the cooked potatoes thoroughly in a colander. Let them steam dry for about 5 minutes. This step is critical to prevent watery mashed potatoes.

Transfer the dried potatoes back into the warm, empty pot. Mash the potatoes using a potato masher or a ricer until they are mostly smooth. Avoid using a food processor or blender, as this can make them gummy.

In a small saucepan, gently warm the 240ml (1 cup) of heavy cream and 120ml (1/2 cup) of milk over low heat until just simmering. Do not boil. Warming the dairy prevents it from cooling down the potatoes.

Gradually add the warmed cream and milk mixture to the mashed potatoes, about 60ml (1/4 cup) at a time, mixing gently after each addition until the desired creamy consistency is reached. You may not need all of the liquid.

Add the remaining 85g (6 tbsp / 3/4 stick) of unsalted butter to the potatoes and stir until it is fully melted and incorporated, making the potatoes rich and glossy.

Gently fold in about two-thirds of the caramelized onions into the mashed potatoes. Reserve the remaining onions for garnish.

Season the mashed potatoes generously with salt and freshly ground black pepper to taste. Start with 1/2 tsp salt and 1/4 tsp pepper and adjust as needed.

Serve the mashed potatoes hot, topped with the reserved caramelized onions. For an extra touch, you can add a small knob of butter on top before serving.