Marriage Is All About Compromise Beef Stroganoff

Name: Marriage Is All About Compromise Beef Stroganoff
Ingredients: 2 tablespoons vegetable (or other high-heat, neutral-flavor) oil, divided; 5 ounces sirloin steak, cubed (½-inch); 10 ounces cremini mushrooms, quartered (stems still attached); 2 tablespoons unsalted butter; 1 large carrot (about 7 ounces), finely diced (about 1 ½ cups); 1 large parsnip (about 7 ounces), finely diced (about 1 ½ cups); 1 medium yellow onion (about 8 ounces), finely diced (about 1 ½ cups); 6 scallions, finely chopped (both the white and green parts); 2 large garlic cloves, minced; 1 1/2 tablespoons all-purpose flour; 2 cups beef or chicken stock (see headnote); 1 tablespoon Dijon mustard; 2 teaspoons apple cider vinegar; 1 teaspoon prepared horseradish; 1/4 cup whole-milk Greek yogurt, room temperature, plus more to taste