For the dark green tops of the leeks, discard any outer leaves that are spongy or beat up, then trim the very tops. Wash well, then thinly slice, and then chop them up some more.
For the bottom half of the leeks, cut them in half lengthwise, then thinly slice. Add them to a bowl and fill with water and swish well to remove grit and let it sink to the bottom. Lift the leeks out (don’t pour) onto a kitchen towel and pat dry.
In a large skillet over medium-low, warm the olive oil and butter until melted. Add all of the leeks and salt and cook for about 25 minutes, stirring every 5 minutes or so, until very soft, silky, and lightly golden. Let cool for 10 minutes – we want them warm or room temp but not hot.
In a medium to large bowl, mix together the yogurt, parm, lemon zest and juice until combined.
Stir in most of the leeks, reserving a couple of spoonfuls for topping. Taste and adjust by adding more salt and/or lemon, if needed.
Spread onto a shallow plate or bowl, swirling to create pockets with the back of a spoon. Top with reserved leeks, herbs, olive oil, black pepper, and an extra sprinkling of parm.
Serve at room temp with crackers, veg, etc…for dipping.