Preheat broiler to high for 5 minutes. Transfer the poblanos to a baking sheet. Cook under the broiler for 5 to 6 minutes per side. Remove peppers from broiler and transfer to a plastic or paper bag to steam and cool, set aside. This is a quick way to roast your peppers, but I still prefer the way Mom did it. Low and slow on the comal(griddle), but if I had a gas stove, I would char them directly in the fire. If you roast them too long in the oven, the stems may come loose and that, to me, is a key part in dipping the rellenos succesfully.
Preheat 1/8 to 1/4 cup of olive oil in a nonstick pan to medium heat for a few minutes. Add the diced potatoes and cook until browned in most spots. I like to cover my potatoes while they cook. The steam created helps them cook a little faster from the inside. Transfer potatoes to a plate lined with paper towels to drain, set aside.
Add the ground beef to that same skillet and heat to medium. Season beef with 1 teaspoon salt, 1 teaspoon garlic powder, 1 teaspoon cumin and 1/2 teaspoon pepper. Cook for 8 to 10 minutes until nicely browned. Add the onions and garlic and continue cooking for 2 minutes. Add tomatoes, serranos and 1/4 cup water to the blender and blend until smooth. Pour tomato mixture into picadillo, add potatoes and stir until well combined. Taste for seasonings and continue cooking at a low simmer until sauce thickens and reduces. Remove from heat and cool.
Carefully peel the blistered skins from the poblanos, cut an opening and remove the seeds and veins. Stuff each one with picadillo, just enough to fill, do not overstuff. Dredge the filled peppers in the reserved 1/2 cup of flour and set aside.
In a medium pan, preheat 2 cups of canola oil to medium heat for 6 to 8 minutes.
Add the egg whites to a glass bowl and using a hand-held mixer, beat on high until stiff peaks form. Tilt the bowl slightly while beating and move the beaters quickly back and forth. This will help put more air into the egg whites.
When ready, add in the egg yolks, 1/2 tsp. of salt and 2 tbsps. of flour. Mix on low just until blended. Dip each stuffed pepper into egg batter. Transfer to preheated oil. Fry for 3-4 minutes. After the first minutes, gently spoon over the top some of the hot oil to seal egg batter before flipping over. Flip over using two flat spatulas. Fry until golden brown.
Drain onto plate lined with paper towels. Serve right away or keep warm in a low temperature oven until ready to eat. Serve with your favorite salsa, Mexican red rice, beans and warm tortillas.