Spinach Soup

Name: Spinach Soup
Ingredients: 2 tablespoons olive oil; 10 ounces fresh spinach; 2 cloves garlic, finely minced; 1/2 medium onion, chopped; 4 tablespoons (1/2 stick) butter; 1/4 cup all-purpose flour; 3 cups whole milk; 2 teaspoons kosher salt, or more to taste; 1/2 teaspoon cayenne pepper, or more to taste; Freshly ground black pepper