Steak-Stuffed Portobello with Creamy Poblano Sauce

Ingredients

• 2 beef Flat Iron Steaks (about 8 ounces each)
• 2 fresh poblano chile peppers
• 1 small onion, cut into 1/2-inch slices
• 4 large portobello mushrooms, stems discarded
• 2 teaspoons olive oil
• 2 teaspoons minced garlic
• 1/4 cup heavy cream
• 1 teaspoon kosher salt, divided
• 1 teaspoon freshly ground black pepper, divided
• 3/4 cup shredded reduced-fat mozzarella cheese
• 1/4 cup chopped fresh cilantro
• 1 tablespoon fresh lime juice
• 1/2 ripe avocado, thinly sliced