In a large saute pan, over medium-high heat, add 1 tablespoon of the olive oil and saute yellow onion, half of the red bell pepper, and serrano pepper for 3 minutes. Add the garlic and saute for 1 minute more. Turn off heat, add in the corn and set aside.
In a medium bowl, mash potatoes with a potato masher, and fold in the sour cream, cream cheese, pepper jack, salt and pepper. Stir in 1/3 of the sliced green onions and combine gently with the corn mixture. Season, to taste, with salt and pepper. Keep warm.
In a small saute pan, over medium heat, add the remaining olive oil. Add the remaining red bell peppers and saute briefly. Stir in 1/3 more of the green onions, then add the tomatillos. Let simmer for 5 to 6 minutes, then puree with a stick blender. Keep warm.
Preheat the oven to 250 degrees F and put a cooling rack on a sheet pan. Heat the canola oil to 350 degrees F. Have tortillas, dish of beans, potato mixture, sliced pasilla peppers, sliced avocado and toothpicks at hand, as well as a plate lined with paper towels for draining.
Put a flour tortilla on dry work surface, lay about 3 tablespoons black beans along 1 side. Top with approximately 1/3 cup of the potato mixture, 2 strips of the pasilla pepper and avocado.
Roll tightly, burrito style and seal with a toothpick. Set aside and repeat with remaining ingredients.
When all are assembled, check the oil temperature and gently add them to the pan, maximum 4 at a time. Do not crowd. Turn frequently and be careful, because they can brown quickly. Drain briefly on paper towels, remove the toothpicks and keep warm in the oven on the cooling rack.
Put 1/4 cup of tomatillo sauce on a plate; cut a flauta in half on the diagonal and lay 1 piece down, and 1 standing up, then sprinkle with Cotija cheese and cilantro. Garnish with the remaining green onions. Repeat the process with the remaining ingredients.
For a main course, serve on large plate, double the sauce, add 4 cut pieces, all up, garnish and serve.