Portobello Mushroom Burger: This Bite-Size Appetizer Will Impress Guests

Ingredients

• 1 small onion (thinly sliced and caramelized)
• 2 tsp vegetable oil
• 2 tbsp fresh chives (finely chopped)
• 1/3 cup walnuts (finely chopped)
• 1/2 tsp salt
• 1/2 tsp black pepper
• 1 4oz package goat cheese (room temperature)
• 4 portobello mushroom caps (4 inches (10 cm) in diameter)
• olive oil
• 8 small dinner rolls ((or 24-inch whole grain baguette, sliced into 8 equal portions, then sliced horizontally))
• green lettuce leaves
• 1 tsp fresh rosemary (finely chopped, for garnish)
• Dijon or whole-grain mustard