In a medium pot over medium heat, add the oil. When the oil is hot, add the onion and cook until softened, about 5 minutes. Add the chicken broth and bring it to a simmer. Add the pork, orzo, and spinach and simmer until heated through and spinach has wilted, about 2 or 3 minutes. Add the lemon juice, taste, and adjust the seasoning with salt and pepper. Ladle the soup into serving bowls and serve hot.