With a vegetable peeler, carefully remove just the yellow lemon zest, avoiding the white sour pith skin.
In a food processor or blender add the Dijon mustard, garlic cloves, herbs, lemon zest, Serrano chile, red pepper flakes and the kosher salt.
With the processor or blender on add the olive oil in a slow drizzle to emulsify and a thick paste has formed. A minute or so should do it.
Make small cuts all over the meat and rub the mixture in the cuts and all over your pork roast. Allow to marinate as you would normally or roast and baste as it cooks.