Trim the woody ends off all the bunches of white and green asparagus and set aside for stock. Then cut about 1 cm worth of thin slices off the remaining asparagus, starting from the trimmed end. Slice until both the white and green asparagus spears reach about 5 inches long.
Reserve thin slices for asparagus purée.
Heat a large cast-iron pan over medium-high heat. Season the asparagus spears liberally with black pepper and lemon salt. Add the oil to the hot pan and let it start to smoke. Add the asparagus spears in an even layer along with any remaining seasoning from the tray. Sauté for 90 seconds or until the green of the asparagus brightens and the white begins to char. Flip the spears, add the butter, and shake the pan so the butter can melt to the bottom. Let the butter brown and the asparagus spears char for another 90 seconds.
Deglaze the pan with vegetable stock. Turn off the heat and place a folded piece of aluminum foil on top of the asparagus to steam for 2 to 5 minutes. Check for doneness with the edge of your knife. If you feel resistance, let the asparagus simmer on low heat for another 2 minutes. If your knife slides through the asparagus fairly easy, remove from heat and reserve for plating.