Summer Squash Soup

Name: Summer Squash Soup
Ingredients: 3 cups yellow summer squash, diced; 1/2 cup chicken stock or water; 1 small onion; 1/2 teaspoon salt; 1/2 teaspoon dried oregano; 2 tablespoons butter; 2 tablespoons flour; 1/2-1 cups plain yogurt; Dash black pepper to taste; Handful shredded cheddar cheese, optional