Bring a large pot of water to a boil. Add in tomatillos, jalapeño, and the chopped onion. Cover and reduce heat to a simmer.
Simmer for 10 minutes, until tomatillos & onions are tender. Drain and carefully put into a blender with ⅓ cup cilantro. Blend until it reaches your desired consistency. Season with ¼ teaspoon salt. Set aside.
Meanwhile, heat oil over medium-high heat in a large skillet or Dutch oven. Add ground beef, remaining diced onion, diced bell peppers, cumin, remaining ¼ teaspoon salt, and pepper. Cook, breaking up beef into small pieces with a spoon, about 7 minutes. Carefully blot with a paper towel to absorb excess grease {discard paper towel}.
Stir ½ cup of the green sauce into the ground beef mixture.
Preheat the oven broiler.
Heat the tortillas according to package directions. Fill each tortilla with about 3-4 tablespoon beef mixture. Top with 1 tablespoon cheese. Roll up, and lay seam side down in baking dish. Repeat until all the tortillas are filled.
Pour the remaining green sauce evenly over filled tortillas. Top with cheese. Broil for about 5 minutes, until cheese is bubbly & lightly browned. Sprinkle with remaining cilantro to serve.