Portobello Burgers with Blue Cheese and Sautéed Red Onions

Ingredients

• 8 large portobello mushroom caps
• 4 cloves garlic, minced
• 3 tbsp. steak sauce (such as A1)
• ³⁄₄ cup breadcrumbs
• 1 egg
• ¹⁄₂ tsp. fresh-ground black pepper
• 2 tbsp. olive oil
• 4 oz. blue cheese, crumbled, for serving
• 4 sesame seeded hamburger buns, split and toasted, for serving
• 3 tbsp. butter
• 1 large red onion, thinly sliced
• 2 tbsp. balsamic vinegar
• 2 tsp. dried thyme
• Salt and pepper, to taste