Kale and Mushroom Croquetas

Name: Kale and Mushroom Croquetas
Ingredients: 3 Tbs. olive oil; 1/2 lb. (250 g) assorted mushrooms, brushed clean and coarsely chopped; Kosher salt; 9 Tbs. unsalted butter; 1/2 yellow onion, diced; 2 garlic cloves, minced; 1 large bunch kale, stemmed and coarsely chopped; 2 tsp. sherry vinegar, plus more as needed; 2/3 cup (3 1/2 oz./105 g) plus 1 1/2 cups (7 1/2 oz./235 g) all-purpose flour; 3 cups (24 fl. oz./750 ml) milk; Pinch of freshly grated nutmeg; 4 eggs; 1 cup (4 oz./125 g) panko (Japanese bread crumbs); Vegetable oil for deep-frying