1 teaspoon vegetable oil
1/2 small onion
4 garlic cloves, crushed
1 3x1" piece peeled ginger
2 1/2 cups low-salt beef broth
1 whole star anise
1 3"–4" cinnamon stick
4 ounces (2 cups) mixed mushrooms (such as oyster or stemmed shiitake), thinly sliced or torn
1 scallion, thinly sliced
Kosher salt
2 packages instant ramen (preferably not fried; noodles only)
1 1/4–1/2-pound piece beef eye round, sliced crosswise 1/8" thick
Bean sprouts, basil leaves, and thinly sliced serrano chiles