Mexican Beef Soup with Beans and Bacon

Name: Mexican Beef Soup with Beans and Bacon
Ingredients: 4 tomatillos, husks removed, rinsed; 4 serrano chiles, seeds removed; 2 garlic cloves; 6 cups homemade beef stock or low-sodium beef broth; ½ cup chopped cilantro, plus more for serving; 3 cups cooked Mayocoba beans or 3 pinto beans (from two 15-ounce cans), rinsed; 4 slices bacon; 8 chiles de árbol, seeds removed; 2 tablespoons vegetable oil, divided; 6 spring onions, halved lengthwise if large, or 1 medium white onion, cut into 8 wedges; 1½ pounds flank steak; Kosher salt, freshly ground pepper; Chopped white onion, sliced radishes, sliced avocado, and warm corn tortillas (for serving)