Place the corn husks in a bowl of water to soften for 10 minutes. Drain and rinse well. Pat dry and set aside.
In a food processor, process 2 1/2 cups of the corn kernels until coarsely pureed.
In a large bowl, combine the pureed corn, masa harina, lukewarm water, baking powder, 1/2 teaspoon salt and oil. Mix until well blended, using your hands if necessary.
Place a dry nonstick frying pan over medium heat. Add the bell peppers, onion and remaining 1/2 cup corn kernels, and saute until tender-crisp, 6 to 8 minutes. Stir in the red pepper flakes and remove from the heat.
To assemble a tamale, place 3 tablespoons of masa mixture in the center of a soaked corn husk. Flatten with your hand and form a small well in the center. Add 1 tablespoon of the sauteed vegetables to the well. Fold the long side of the corn husk over the filling to cover, then fold over the ends, overlapping them. Tie with a thin strip torn from an extra soaked husk. Repeat to make 18 tamales in all.
In a large pot fitted with a steamer basket, bring 2 inches of water to a boil. Layer the wrapped tamales in the steamer basket. Cover with a damp kitchen towel and steam until the filling becomes firm and the tamales are heated through, 50 to 60 minutes. Add more water as needed.
While the tamales are steaming, make the salsa. In a small bowl, combine the avocado, tomatillos, lime juice, cilantro, jalapeno and 1/4 teaspoon salt. Toss gently.
To serve, place 3 tamales on each plate. Accompany each serving with a generous spoonful of salsa on the side.