Step 1: Cook the Potatoes

1. Boil the potatoes: In a large pot, add the peeled and chopped potatoes and cover them with cold water. Add 1/2 teaspoon of salt. Bring to a boil over high heat, then reduce to a simmer and cook for about 15–20 minutes, or until the potatoes are fork-tender.
2. Drain and set aside: Once cooked, drain the potatoes and set them aside.

Step 2: Caramelize the Onions

1. Heat oil and butter: In a large skillet over medium heat, add the olive oil and butter.
2. Add onions: Once the butter has melted, add the sliced onions and sprinkle with the remaining 1/2 teaspoon of salt. Cook, stirring occasionally, for about 15–20 minutes or until the onions are golden brown and caramelized. If they start to stick, you can add a splash of water to deglaze the pan.

Step 3: Mash the Potatoes

1. Combine ingredients: In a large mixing bowl, add the drained potatoes, caramelized onions, sour cream, milk, black pepper, and half of the Gruyère cheese.
2. Mash: Use a potato masher or electric mixer to mash the potatoes until smooth and creamy. Adjust the milk to reach your desired consistency.

Step 4: Add Cheese and Serve

1. Mix in cheese: Stir in the remaining Gruyère cheese until melted and well combined.
2. Garnish and serve: Transfer the mashed potatoes to a serving dish, garnish with chopped chives or parsley if desired, and serve warm.