Beef & Mushroom Stroganoff With Creamy Polenta

Ingredients

• 2 1/2 teaspoons kosher salt
• 2 cups coarse cornmeal or polenta (not instant)
• 4 tablespoons extra-virgin olive oil
• 2 Spanish onions thinly sliced
• 1 large shallot thinly sliced
• 1 tablespoon minced garlic
• 3 sprigs fresh thyme
• kosher salt and freshly ground pepper
• 4 cups sliced mushrooms (such as oyster, maitake, cremini or shiitake)
• 12 ounces beef sirloin, thinly sliced (ask your butcher to slice it for you)
• 1/4 cup brandy
• 1 cup beef broth
• 2 cups packed baby spinach leaves (stems trimmed)
• sour cream (for serving)
• chopped fresh parsley (for garnish)
• grated lemon zest (for garnish)