How to Make It
Heat a Dutch oven over medium-high. Sprinkle beef with 1/2 teaspoon salt. Add oil to pan. Add half of beef; cook, turning occasionally, until browned, about 6 minutes. Transfer browned beef to a plate. Repeat procedure with remaining beef.
Add garlic to drippings in pan; cook, stirring constantly, 1 minute. Add 3 cups stock, scraping bottom of pan to loosen browned bits. Return beef to pan; stir in cumin, oregano, and bay leaves. Bring to a simmer over medium. Partially cover; simmer 1 hour.
While beef cooks, bring remaining 3 cups stock to a boil in a saucepan over medium-high. Working with 1 chile at a time, hold over a blender; gently pull stem to remove, letting seeds fall into blender. Discard stems; place chiles in blender. Pour boiling stock over chiles. Cover with blender lid; let stand 30 minutes. Process until smooth, about 1 minute. Set aside until beef has finished simmering.
Add chile mixture, squash, and remaining 1 teaspoon salt to beef in Dutch oven. Simmer over medium, partially covered, until liquid is thickened and beef is very tender, about 45 minutes. Sprinkle with cilantro, if desired.