Slice the leeks into 2.5 cm rounds, then rinse under cold water, checking between the layers for dirt and sand. Transfer to paper towel and pat completely dry.
Heat avocado oil in a large oven-safe skillet over medium-high heat. Reduce the heat to medium just before adding the leeks.
Add the leeks and sear for 3-5 minutes on one side until golden brown. Season with salt and pepper.
Flip the leeks, season again with salt and pepper, and sear for another 3-5 minutes until golden brown.
Turn off the heat, then use tongs and a paper towel to soak up as much frying oil as possible.
Return the skillet to medium heat and add the butter, garlic, herbs, and another pinch of salt and pepper.
Heat the butter until foamy and hot, but do not brown it. Baste the leeks with the butter so they soak up the flavor.
Pour in the beef stock, turn off the heat, and transfer the skillet to the oven for 15-20 minutes until the leeks are very tender.
Sprinkle with flaky salt before serving, if desired.
Notes
Leeks hold dirt between their layers. Cut them into rounds first, then rinse each round carefully under cold water, especially the upper green layers.
Store leftovers in an airtight container in the fridge for up to 3 days. Reheat in a skillet with a little more beef stock until warmed through.