For the herb sauce: Whisk together the herbs, olive oil, vinegar, serrano, garlic, shallot, salt and pepper in a small bowl. Set aside.
For the three cheese waffle hash browns: Preheat a waffle iron to medium.
Toss the shredded potatoes in a bowl with the melted butter, the three cheeses and the salt and pepper. Mix until well combined.
Spray the top and bottom of the waffle iron well with cooking spray. Put a mounded cup of the potato mixture (about 1 1/4 cups worth) in each well and gently pat down. (They will start to sizzle.)
Close the waffle iron and cook until the hash browns are golden brown and crispy; time will vary based on your waffle iron, but it will take roughly 12 minutes. (Check on them occasionally to make sure they aren't burning.) Use a butter knife or fish spatula to gently and very carefully remove the hash browns. They are very fragile when they are hot and just out of the iron.
For the flank steak: Meanwhile, preheat a grill or grill pan to medium-high. Pat the steak dry and sprinkle liberally with salt and pepper. Grill the steak for about 5 minutes on each side for medium-rare. Set on a cutting board to rest for 10 minutes.
Thinly slice the steak against the grain. Divide the hash browns and steak among 4 plates and drizzle the steak with the herb sauce.