Trim the roots and dark green tops of the leeks, slice them in half lengthwise, and clean thoroughly under running water. Pat dry and set aside.

In a large skillet, heat olive oil and unsalted butter over medium heat until shimmering. Place the leeks cut-side down in the skillet and sear for 3-4 minutes until light golden brown.

Pour in dry white wine and white wine vinegar, followed by vegetable or chicken stock and minced shallot. Cover the skillet and reduce heat to low. Braise for about 25 minutes until tender.

Remove the leeks and keep warm. Bring the remaining liquid to a gentle boil, whisk in heavy cream if desired, then add cold unsalted butter cubes, whisking until smooth.

Stir in lemon juice, salt, and white pepper, then arrange warm leeks on a platter and drizzle beurre blanc over. Garnish with chives or parsley and serve immediately.